damnyankees: Well, you basically need a lot of Tourism. Not sure how easy it is to get, but the trade routes are a universal bump to it. I think the Hotel link is the best way of working it.
blackcatatonic: Heh, I'd love if they got a free MoV and (especially!) 50% reduction in their cost in the capital!!
Still, though, I think their non-militant takeover of CSs with MoV is a red herring. Venice is ideally suited to a warmongering infinite puppet empire, particularly on aquatic maps, and so they'll want to maximize their trade routes with CSs...
Incidentally, I greatly prefer the peaceful culture game, so I find it ironic that I can't view Venice as anything but a warmonger Civ.