Off-topic, but I was thinking the 3g/4g had little to do with voice calling, and had much more to do with data. The Vita doesn't have the regular cellular radios needed, unlike say an android phone. Even most tablets have to do VoIP rather than straight calls.
Now, a Vita/Xperia hybrid might not be a bad idea down the road, but I doubt it would run Android. Android's too buggy and fragmented to be stable enough to power both the phone and the Vita software. Sony might have to make a dialer/phonebook program that runs with the SDK, and I don't know if they'd make much profit off of it.
Plus, you'd probably be locked to AT&T in the US, which is a definite negative.