Lots of research. Read reviews, and look for social media accounts and talk to them a few times before booking. Make a list of questions to ask and interview them. Tell them you're checking availability and pricing before making a decision. I've used the bash (aka gig masters), thumbtack, gig salad, wedding wire and the knot to search for suitable options. Also, vendors often meet working the same events together so vendors can recommend others (or warn you when to steer clear) so ask for recommendations. Hope this helps, good luck!
One of the best ways I vet vendors - ask other vendors! Not judgmentally, but just a "Who do you recommend for (need)?" and they'll usually happily give you a list! If they haven't worked with someone they'll usually tell you as much.
I've discovered that reading reviews on numerous platforms (Google, The Knot, WeddingWire) and asking for recommendations from previous clients are really helpful. Joining wedding planning Facebook groups has also been really beneficial in terms of receiving genuine comments. What types of vendors are you looking for?
Re: How do you vet vendors?