Just been debating this with myself. Given that price isn't an issue, what's important to you?
Two of the most important traits in a vendor for me are good communication and good stealth. The reason for stealth is fairly well discussed.
With communication. Among other factors, it put me off one vendor for good, although he's not in business anymore. He didn't respond very quickly regarding shipping time concerns. For another their poor communication has cost them part of the order I'm about to make with them. Their profile clearly states they do custom listings and I contacted the vendor regarding this. I wanted a specific quantity for one part of the order and was trying to find out what he would offer for the other part of the order given I'd pay xBTC. He says he doesn't do custom quantities for the part I wanted a specific quantity which I accepted but, apart from the fact he isn't quite truthful in his profile, he also isn't very speedy with his replies. I last heard from him 6 days ago. Sent two messages in that time and frankly there's another vendor I've dealt with before who I can use again for the second half of my order who's very accommodating and also replies promptly.
Bad reviews...plain and simple. As long as it covers what a review should cover - as outlined in most vendor reviews around here - you can get a clear enough picture of how that vendor operates. Reviews on the forums as well. Good communication is like a cherry on top, especially from a reputable vendor doing a shit ton of business...and by good communication, I'm talking about above and beyond. Some vendors have an endless supply of patience (for multiple questions) and others don't. Most vendors these days recognize good customer service is the way to make money. Have shitty product that everyone recognizes as shitty? Shipping time lasting far too long without any explanation? Questions easily answered by any cogent review.