You'll Only Be Able To Download About 20MB Of Data Via PS Vita's 3G Connection.

The publication cites an unnamed Sony representative as the source of the information.

Of course, a cap like that really limits the potential for 'on-the-go' downloads. Asynchronous gameplay data, save files and (potentially) Minis would be ok, but not a lot else. ASCII's source did stress that the limit could increase as the Vita matures, but the reality is that it's never going to be great enough for full game downloads.