Yea, the A330 300 seat selections are not available yet, but should open hopefully soon.
However, this year, my family and I had problems prebooking our seats, out and return, I ended up selecting a group of 3 seats and 1 on its own with strangers for our outbound flight, hoping that if we got to check-in early we would be able to change and get 4 seats together, as Virgin only allow prebooking of about 60% of seats on plane.
How wrong was I, we decided to try to check in online, however due to system fault this was not available, so we made sure we were at check in as it opened to get best chance of changing, only to be told "Sorry no seats available together" I said but you've only just opened and you only prebook 60%, The check in agent then went on to say "we have an automated system that allocates everyone that has not prebooked into seats approx 2 days before departure" this means whether you get to check in early or not, if you have not prebooked seats your seats will already be allocated to you by an automated system.
For the return flight I didn't have any option to prebook any seats in advance, so as normal we checked in at Downtown disney, and hey presto, as there was 4 of us, we got 4 seats together.
So for future reference, beware of this system of seat allocation.