I hate to post this here, but there is an alternative to Vocito that
does integrate with Google Voice.
I've been using Dialectic for a couple of days now and it does work
with Google Voice. That said, I will switch back to Vocito immediately
if it becomes compatible... Dialectic is feature bloated, and the
address book functionality is not nearly as elegantly implemented as
it was in Vocito. Also note that though it comes with a free trial, it
does cost money.
Hope this helps!