Cross-buy is exclusively a Sony first-party thing, because they're the only ones who care about selling Vitas. They'd need to pay out the ass for other publishers to give away free copies for no benefit.
It would be to Sony's benefit to pay developers to make this happen more often though.
But awesome as it may be, I don't think DC is going to be a huge system seller. Which is too bad really, since the Vita is a pretty sweet little system. Needs to be reworked to allow using multiple regions' PSNs without doing a system restore, though. How the hell else am I to get DLC for games I imported? :(