Same as you would anyone else. You tell the VC that you can't discuss the details. If they want details they have to sign the NDA for the 3rd party company.

Same as you would anyone else. You tell the VC that you can't discuss the details. If they want details they have to sign the NDA for the 3rd party company. You can always talk about the commitment in general terms (revenue vs. time spent) and if it's strategic to the growth of the company. The rest of the details don't matter much unless the 3rd party is in their portfolio or a competing portfolio.