I don't think this is possible as VS lacks the IT capability to book with the voucher for the outbound (or indeed at all).
Thus you need to call VS anyways to book. I would thus ask about this if you sense you have a good booking agent (and 80%+ are in my experience with Virgin Flying club).
I do suspect this is a scenario where it MAY pay to only use the 2f1 for the outbound, and suffer the extra points inbound but save on the 'carrier imposed surcharges' from GRU.