The V-DAC is a good product, but I disagree about the comments on the Decco/iDecco. In the setup you're describing, the Decco would be better in my mind. Unless you're expecting the amp to sound as good or better than a $5-600 amp, the amp isn't complete rubbish. It's not the most detailed or dynamic amp I've ever heard, but it's balanced fairly well and it's plenty musical. Also, the preamp is pretty good. You can get the newer iDecco for ~$800 right now, and I can't think of one preamp I've heard in that price range that I would definitively take over the iDecco. I think you can find a used preamp for the same money that's better, but that's a different story. I like it as much or more than anything I've had from NAD. I actually do use an outboard amp, but that's mainly because I've been in the mood for some tube rolling lately.
All in all, if it would be replacing an older receiver or you'd be going with NAD and the V-DAC, I have to believe, that you'd be just as happy (if not more) with the Decco in that setup.