This may not be the 'how to' you are looking for, but it is the one lesson after many years of buying vintage that I have learned.
It isn't about what the piece is worth, or its resale value, it is simply because you want it. It is the fact that right in front of you is the thing that you want at the price you are willing to pay. That's it. For each person it is going to be a different thing and a different price and even different condition. I don't mind if something is a bit scratched or dinged, but you may want it in pristine condition (which will have an impact on the price FYI!). If you adore it/love it/want it/feel like a part of you would cry if you walked away then buy it.
There are so many ways to mix vintage pieces into your home. Even if it isn't your style, don't worry. Homes are meant to be a mix of pieces acquired over time. I mix Victorian and mid-century modern and it just works.
So if a piece makes you happy and you can afford it, then buy it.