I agree with others who say it's better not to go overboard with trying to create an accent on paper. First of all, it's hard to say which pronunciation of a word is most phonetically "neutral" anyway, and it's hard to agree upon spellings of given sounds. Think of all the words in English whose spellings don't follow "standard" rules of pronunciation in any English dialect or accent. There are international rules of pronunciation, but you'd need to use all those symbols (as in a dictionary). People with different North American English accents spell English words the same, so we have different views on how certain letter combinations should be pronounced. Which accent is truly "neutral" English, even within a given country?
Secondly, it can be annoying to see spellings distorted over and over. A few writers have a knack for it (usually when shooting for humor), but if it's done badly it can put readers off or even be insulting to people with particular accents.
Thirdly, in your specific case, if your characters are Vikings, I assume they would really be speaking in their own language and are in an archaic setting, which means they wouldn't have an accent as far as they're concerned. Then you're really translating what they're saying into modern English anyway. Bernard Cornwall has his characters speaking modern English in his historical novels (I I recall one with Saxon raiders and so on).
You can still choose to represent certain turns of phrase differently between different groups of people, of course, or to describe how their accent sounds to someone who has a different one. And when writing in historical settings, or in fantasy settings that are pre-industrial, many authors choose to omit words and turns of phrase that are anachronistic (like someone wouldn't "escalate" a situation, nor "slam on the brakes" nor "go off the rails" etc.)