Sounds like you got an excellent price. I cannot comment on Travelcity as we have never flown with them but I do like Virgin.

As for booking last minute or well in advance, there are advantages and disadvantages with both. I personally like to book everything early. I don't like to leave it too late in case I cannot get the flights I want. Sometimes you can book a Virgin flight for less than £300 a few weeks before departure but I think that it would still come to more than £1000, for the four of you, as that does not include car hire.