After 30 years studying martial arts - one of the skills that develops is a great recognition of people. they say don't judge a book by it's cover - and it's mostly true - but the vibe people give off has proven itself over 90% accurate in my experiences.
No such thing as a vibration your 'body' picks up on. Knowledge, and experience triggers you mind to send the 'watch out' feeling.
People often forget that humans are animals too, and we too have instincts which enabled us to survive and recognize danger. most peopel discount their gut feelings as too emotional. However, I have regetted every time I have gone against my gut feeling. I hve nto regrets about following my gut instinct.
Read, 'Women's Ways of Knowing.' Great book!!!!!!
Dawkins talks about gut instinct in a few of his books on evolution. I think it's important to understand that while it is entirely irrational (or perhaps arational if that can be used as a proper word), gut instinct exists because our it saved the lives of our ancestors. I take such feelings as a firm warning to be cautious. If I can't get to the bottom of the feeling, I'll bug out.
I do not believe in chemistry. This has been discussed many times here. I do believe in one's ability to perceive certain body languages. Some have better relationships because they can read another's body language and some, like me, cannot and have a harder time. That is why I like to get to know someone first through writing or phone calls.
I don't think it's really a bulletproof way to do things, no. Have I had intuitions it turned out to be correct? Absolutely. Have I been totally wrong as well? Absolutely. I would never trust my intuition alone in all circumstances.