How are you going to know they betrayed you?

If you know, how are you going to prove it?

If you can prove it, how are you going to litigate it?

NDAs only real usefulness is for big players to legally bribe and contain small fry.

Asking for an NDA from an early stage VC is likely to actively hurt as it can make you seem inexperienced in the realm of startups/venture capital. A decent amount has been written on this. Here's a couple examples, but there are others: http://www.paulgraham.com/start.html https://feld.com/archives/2006/02/why-most-vcs-dont-sign-nda...

They literally won't sign an NDA, especially at that early of a stage.