Unless things have changed since I was an intern at a VC fund almost a decade ago, VCs rarely sign NDAs, especially before due-diligence is complete. There are a lot of good reasons for that, and I've since found it a good personal policy when working with potential clients - the legal liability incurred by signing an NDA that covers something you don't really know the scope of isn't usually worth it.
I hate to beat the 'all ideas are worthless, only implementation matters' drum, but a lot of ideas are pretty worthless. If I had a dollar for every time someone tried to get me to sign an NDA for an idea that was literally "Build a web page that we can charge people to list their website on.", well, I'd have a lot of dollars. People are generally a lot less clever thank they think they are.
That said, I don't think it is too terrible to sign a limited-scope NDA once you have a general idea of what information you might be coming into contact with and are ready to actually start working. There are very few ideas that are so powerful that you can't at least have some high-level discussion about prior to signing an NDA, but there are certainly a lot of details worth protecting in some cases.
I hate to beat the 'all ideas are worthless, only implementation matters' drum, but a lot of ideas are pretty worthless. If I had a dollar for every time someone tried to get me to sign an NDA for an idea that was literally "Build a web page that we can charge people to list their website on.", well, I'd have a lot of dollars. People are generally a lot less clever thank they think they are.
That said, I don't think it is too terrible to sign a limited-scope NDA once you have a general idea of what information you might be coming into contact with and are ready to actually start working. There are very few ideas that are so powerful that you can't at least have some high-level discussion about prior to signing an NDA, but there are certainly a lot of details worth protecting in some cases.