A lot of VC communication is about trust and your ability to work together, tempered with commercial common sense. You absolutely need an NDA and to draw your own lines in advance about where public knowledge ends and your IP starts. That may well be at a higher level than you think, since what's common sense to you with your experience is a genius idea to the uninitiated.
However the relationships are often very much worthwhile. If you're overly concerned, you can set boundaries around your IP prior to the meeting so they know what is on the table for discussion. It's a perfectly reasonable position to take.
I highly recommend you watch Silicon Valley season 2, episode 10. "Runaway Devaluation".
They show a similar situation to yours.
And they have a two-word name for what the VC appears to be attempting: the first word is "brain" and the second is a 4-letter word that starts with "r" and ends with "ape".
The short answer is: just say "no, thanks".