Just a shot in the dark as I have never tried it, but logically I think this should work. Have all extensions in a cover group. Have that cover group assigned to a calling group that immediately overflows to the vmail calling group. Create 1 mailbox with the same number as the first calling group. You would need to test this, as I don't know if the integration to vmail would in fact be the 1st calling group extension or the original extension number.