"Vetting" is a dictionary word, but in my own words, means to figure out what you are able to know about a person and if, given what you're confident in knowing about them, whether you can trust them. You want to talk with them a bunch before you agree to meet, and also what @theblueone suggested about meeting for lunch first has been my offer on many first meets and would seem to be a successful strategy. I'm sorry it hasn't worked out so great for you so far @theblueone.
What to look for in your pre-meet conversations? Anything that feels uncomfortable. Anything that sets your "Spidey-senses" a-tingling. You are going to one degree or another make yourself vulnerable to this person (even if you are expecting to be the ler). @dvnc makes an excellent point that most reputable gatherings "vet" their attendees as well as provide monitors to make sure nothing gets out of hand. If you are meeting someone on your own, you are taking all of that on yourself. I'm not discouraging that, just use your whole brain when evaluating the situation before you meet.
I've had one gathering under my belt, which was amazingly positive, but that one won't happen again. Many many more meetups I've taken upon myself and enjoyed the heck out of them. I'm not an expert in any way, but anyone that wants to ask me any questions, please feel free to ask here or DM me. I will always encourage anyone to seek sources besides myself for any important decisions. In that same spirit, I encourage anyone experienced with meetups to chime in and contradict me or agree or whatever you feel is right 🙂
//R