I'd say:
Before me was a tall double door, flanked on either side by two large, blue vases.
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Couple points:
- Though it is technically redundant, it helps me MORE, as a reader, to have "flanked" AND "either side" here. Again, it's technically incorrect, but I think the author was wise to put this, and "felt" this one correctly. (besides, you have to specify that you aren't just flanked on ONE side, which is the default assumption when you say that something flanks you)
- As a reader, I couldn't care less about having a passive voice here. There is absolutely no loss of effect, for me... (in fact, the only loss would be the poor writer having to stop and waste her time*thinking* about these details)
- I disagree with IceCreamEmpress that you don't need a comma after "large" in "large blue vases" just because size and color are different types of attributes. As an example, look what happens when we extend that description with new attributes, and don't use commas: "The man was flanked on either side by two large blue smooth-feeling tilted ghostly vases." See, even though each attribute is a different type, it doesn't mean we don't need commas.
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All-in-all, I think Petronella got more than she bargained for, with all the analysis in this thread!
If I were an editor, I would have only dropped in a couple commas for you in that line... that's it. I don't think it's worth any thought.