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...