Ask HN: How do you handle NDA'd projects and VCs?

I know VCs don't generally sign NDAs, and that your idea isn't so special, etc.-- but how do you handle discussions with VCs that are about work you're doing for an external client that's highly confidential, such that it wouldn't be the startup's call to disclose it? If it's a source of revenue and a major consumer of engineering time it would obviously be of interest to the investor, but you're under NDA and can't discuss it. What do you do in this situation?