Is this cheating?

I'd happily go along with this setup as long as the potion makes sense in the world you create. I quite like the idea of watching the world progress as they sleep.

I'd read it, too. Among other things, the idea has a lot of potential for humor. You say the potion gives them information about things going on around them, but how does the potion decide what's relevant and important enough to show? I doubt it would make them omniscient (or does it? Are they familiar with the life story of every person who was born and died within the past 200 years? Animals? Plants? Geological shifts? Are their brains so stuffed with irrelevant information that they have trouble focusing on the here and now?) How does the potion know which modern language they need to pick up? Does it ever give them incorrect information? What about information that's correct, but counter to what most people believe to be true? (That would probably get them labeled as conspiracy nuts).

Seems like the potion will probably have some parameters to answer these sort of questions, and the minute you have a rule you start running into exceptions. Maybe the potion shows them major political events, but fails to mention how to work a coffee maker, or vice-versa.

Where does this take place? Because I can't imagine them keeping up with everything, all around the world, but I would certainly believe them having retained some level of conciousness.
So they can 'see' and 'hear' what's going on near them, what people are talking about, what images people are looking at, but not the details. They can recognise things, but not the whole story.

If you put them in a big city, they would get a lot of information over 200 years. Less so in the crypt of a rura; church.

So the potion tells them what's new... does it also tell them when it goes out of fashion? Because while you want them to blend in, there's a lot of potential for humor with them seeking out something that went out of style 100 years ago.

And does it explain how common/uncommon/expensive things are? If they're shown trains, buses, airplanes, and spaceships, for example, would they be aware that it's simple to purchase tickets to ride the first three, while the fourth is functionally impossible for most people to obtain?

Oh, and what about social changes? Slavery wasn't abolished in the US until 1865, and women didn't get the right to vote until 1920 (those two items alone changed society drastically. Obviously there are many, many others). However, POC and women obviously existed in their time, and explaining major societal shifts using only relevant vocabulary lessons would be... tricky.

This concept is a hard sell to me with the short description so far. It seems like the need to filter and limit the scope of the information coming in would need a lot of specification, almost like specific programming. Scientifically it brings up questions about how the brain is perceiving all this information without any energy input for the body for 200 years, but this point is more easily put aside when magic is the cause. There's certainly a lot to consider to make this work seamlessly.

Yes, it's a hard sell for me too.

Why not give them a magical thingagamuffin to use when they wake up so they can catch up on the world?

It's a bit of a pass for me as well. By reading the setup I'd assume the magic system used to create this omniscience sleeping potion will also act as a Deus ex Machina at difficult moments of the story. I'm perfectly alright with magic, potions, or whatever new mcguffin an author comes up with existing in a world, but, for me at least, there has to be limits and a cost for the use of powerful magic. Sort of like the time-turner magic of Harry Potter, it left me wondering why in the hell didn't someone use a time turner to go stop Voldemort when he was just a disturbed youth? (Looking at you Albus!)

I'd also think to myself if vampires needed to lay low for a few centuries and there's a potion that can make them sleep, and awake with full cultural knowledge around them. I'd wonder why there wasn't a potion to let them disguise themselves as humans so they could age, go into the sun, eat regular food, not need as much blood, or suspend whatever characteristics your particular vampires have (maybe they're not shiny or whatever) and eventually appear to die. Then they revert to their original vampire form and must take another potion or find some new ingenious way to lie low. Which will largely depend on the rules of your world.

I had a MS I was working on for a bit where vampires could sense others of their kind within about a kilometer. But a vamp could go undetected to within a few feet if they fed only on the blood of newborns for a month. That gave me a trade-off that tempted my MC when he needed to hide, but he refused to pay the cost .

The Second Moon said:

Okay, so I need a consequence. What if it made someone they both cared about die? The MC's best friend would be a perfect candidate, because he needs to be a ghost in the future anyways. But of course the MC and his wife wouldn't know about this consequence, because they would never let their friend die.

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Tie them together: the potion works by creating and enslaving a ghost (a.k.a killing a person), tethering them to the mortal realm and "feeding" the two information on the passage of time. Whether or not the two (or one of the two) know or care how the potion works, or who the victim would be, could create some nice conflict and friction. And it could also explain some gaps in knowledge; they'd only know what the ghost "witnesses" and reports, and if the ghost is physically bound to a given area there will obviously be gaps, especially before mass communication.

Brightdreamer said:

Tie them together: the potion works by creating and enslaving a ghost (a.k.a killing a person), tethering them to the mortal realm and "feeding" the two information on the passage of time. Whether or not the two (or one of the two) know or care how the potion works, or who the victim would be, could create some nice conflict and friction. And it could also explain some gaps in knowledge; they'd only know what the ghost "witnesses" and reports, and if the ghost is physically bound to a given area there will obviously be gaps, especially before mass communication.

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This is a very good idea. Without this I would have said, no, seems too unbelievable and trite. I like magic in stories to have some sense - a framework for how it works, rules and limitations. The initial description of the potion seemed like a child's airy-fairy idea of magic that can do anything at all. A telepathic link to a person or ghost so that they simply learn what the person/ghost sees, hears and perhaps thinks is quite believable (as fiction).
What if the best friend has to drink a potion too which binds them. They understand that after the friend's death he will hang around as a ghost (essentially tethered in this world by the mental connection formed by the potion). Perhaps they assume he will live out his life as normal first (while feeding them info) and don't realise the potion saps his life soon after the couple are asleep. The couple could be aware he is dying but unable to wake themselves up in time to save him, or perhaps he dies quickly and the couple regret what they have done but there's no point waking up now and wasting his life. Does the couple then have a telepathic connection with each while asleep - can they essentially communicate? Or is it like a deeper sleep where the sensations they receive from their friend are like a dream, simply receiving without active thought. 200 years is a lot of info. I imagine they wouldn't remember everything clearly as soon as they wake up. Ever woken without remembering your dream until something during the day suddenly reminds you of a dream or fragment of one? - I imagine it might be like that at times; that when they wake they would be conscious of a lot but some memories would only come later.
Then what happens with the ghost and telepathy once they are awake? Does waking naturally break the link? Is another potion required to break it, or would you have them continue with it? Perhaps it fades away over a few days or hours, their fully conscious brains unable to deal with input from the friend's senses as well as their own and the link is rejected like your body forcing out a foreign body (eg splinter). How do they wake up? Does the potion have a built in alarm clock or does the ghost have to do something?

I did also wonder - does he assume there will be no vampire hunters in the future? Were all other vampires wiped out in the past so hunters not needed? Is this couple the only ones that have ever used this potion? How do they get it, make it, know of it when it's presumably not being used by other people or vampires?

I think it's a great idea!

Full sails ahead Second Moon!