A350 capacity wise may be too big. Virgin are not in the best place financially, surprised at the lack of take up on some of the new MAN long haul. Perhaps TCX is just a better fit for the MAN market?
As far as I know, VS have 10 identical A330-343Es so all capable of MAN-SFO. However if BOS/SFO do not make targets, easy enough to rebook existing bookings over ATL/JFK direct from MAN now.