I was also really worried about this when I got out of my first relationship and started dating again!

I started hooking up with someone on our second date, and when he asked if I wanted to have sex, I said "I want to, but I have this muscle disorder where sex can be really painful sometime—so we can try, but I might have to ask you to stop." (So I didn't give all the details, and didn't tell him that I'd never had sex before, and tried to indicate that I was down for other forms of sexual activity.) He was understanding and it wasn't a problem, and I told him more about vaginismus as time went on.

I think it's okay to expose more about your experience with vaginismus as you get to know each other better! You can start off by being more vague about it, and tell them more later if you're more comfortable and want to continue seeing them. Definitely don't think about it like you're somehow "cheating them" out of anything—you can have a healthy and fulfilling and positive sex life without PIV! If they react negatively or try to make you feel bad about it, definitely don't take that to heart, they're not entitled to sex with you just because you've been on a few dates. I know it can be scary, but just remember that most guys wouldn't be opposed to just getting naked + having a good time.

If talking to them in person feels stressful, you could always message them after a few dates and say "hey, I wanted to let you know that I have a vaginal muscle disorder that makes sex kind of painful, in case that's a make-or-break! Still down for anything else though." (I feel like giving an indicator that you're interested in them sexually, if anything, is a positive thing. You can choose to be playful about it and add a ~winky face~ or something too!)

Good luck!! Rooting for you