You really want to have a complete player at the catcher position, and while it is important, it isnt critical. Hell, Posada has been a full time catcher since 1999, and all we've done is win 3 championships and go to 5 world series in that time. And he's about as bad as they come in receiving. BUT, his offensive skills far outweigh his defensive deficiencies, making him more than worth it.
VMart isn't a terrible catcher from a receiving standpoint. Maybe he doesnt call a good game, but that's on Francona if thats the solution. Then Terry can take it from him and call the pitches from the bench. But he is absolutely horrible in the throwing game. Will it kill the sox? Probably not, as long as VMart hits