Should VCs sign an NDA?

Legally: Yes
Practically: No. Most big investors (angel investors, accelerators, VCs) are offended by that question. And, they almost never agree to sign an NDA. If there is an invention that you want to protect, then I would file a provisional patent application before pitching it to the investors. Filing a provisional patent application will give you "Patent Pending" status and that might make you look good in front of the investors.