Actually, this kind of does explain how Roxas is apparently able to feel emotion. Potentially, when he was formed, either a part/the entirety of Ven's heart (minus his memories) is transferred over with him. I'd be tempted to say that he's a Nobody of Sora with Ven's heart, but then that doesn't really explain why he looks like Ven. Unless of course that just a physical body is required to make a Nobody, and the appearance depends on the heart.
We could look at it another way; rather than Ven's heart being a singular thing in Sora's body, it's become more of a "part" of Sora, ie. integrated into his being. If that's the case, it would sort of explain both why Roxas looks like Ven (because Ven is a part of Sora, rather than separate), and that along with the physicality of Ven, some of his heart is transferred over too.
In short, Sora's Nobody, whilst theoretically being entirely a part of himself, is the embodiment of another (technically still existing) person, heart (or vestige of) and all.
God damn, Nomura.
EDIT: Just realised that Days is actually a bit more relevant to BbS than first impressions would suggest, if the above happens to be the case. I wonder if the mentions that Roxas was "special" were subliminal hints.