Sorry to be negative on this one but, in eight flights with Virgin, we have been given our requested seats only twice, and that was the time there were only two of us travelling. On the last three trips (six flights) I have called exactly 180 days out to make our requests.
(To be fair to Virgin, I'm not sure that I actually made any requests the first trip so it may be two successful requests out of six, rather than eight.)
We returned yesterday from our most recent trip and didn't get the requested seats on either leg, although on the return flight we did check-in very early and managed to get two bulkhead seats instead of the alternatives we'd been assigned
On the way out I was asking the girl at check-in why Virgin bothered with seat requests when, in our experience, they were so rarely successful requests. She didn't know. She checked our return flight details and could tell us there and then that we hadn't been allocated our requests for the return flight either.
I remember queueing behind two women checking in with Virgin at Orlando once and heard them asking if they could be allocated two of the rear seats where the rows change. The check-in girl explained that they had all been allocated. They persisted and kicked up a bit of a fuss, and the two seats they wanted magically became available right before their eyes

. Whoever had requested them obviously didn't get them that time.