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.

The reason (valid or otherwise) that VCs decline to sign NDAs is that they may be evaluating several companies in the same space and don't want to be legally constrained from investing in others if they pass on your concept if it is too similar. However, hearsay is that some bottom feeder private equity firms if they like the idea and not the team, finance a fast follower (entrepreneur in residence) using the market insight and prior negative knowledge (ie avoid all the failures). IMHO such IP vampires should be staked and publicly exposed because they distort the risk-reward equation of private initiative. Generally, if you ask around, you get a sense of which firms are credible and have street cred. What is valid contract-wise is when it gets to a stage where there is serious due-diligence, then you can ask for enforceable undertakings, eg make a decision within x weeks in return. Note that as OWNER, the external investor is considered by law as having the same rights and privileges as the original founder so they should all share the same information. If you are treating your investors as second class citizens, then I think there are bigger unresolved issues. Some say getting SeriesA is like marriage so you have to have a degree of confidence in each other.