First, dont get too excited about "pre-booking" your seats. It is a common misconception that you are actually "BOOKING" your seats, when all you are in fact doing, is REQUESTING your seats. Many, many people dont realise this (and I blame the airlines who dont make it clear enough, though there is small print which explains it

). So, even if you "request" your seats, even up to a year in advance, there is absolutely no guarantee that these will be the ones you will get.
Yes, you can book online, but Virgin Holidays customers sometimes will need to call as the reference numbers often dont work on the online system.
You need to go to the Virgin Holidays website (or
Virgin Atlantic, if you have booked a flight only) and go to "Manage My Booking" (for VH) or "Manage My Flights" (at VA). You will need to input your reference and or booking numbers to access your flights. You will then find an "edit" command which lets you "request" seats and meals.
You dont need your ticket, the number from your Invoice is fine (or eticket reservation number). As I mentioned before though, it is not uncommon for these not to work and you may have to call them to do it.
Good luck.
Allie