For security reasons, since Sept. 11th, VS don't make public in advance which aircraft will be flying a particular route on any given day.
You can tell which type of aircraft will be flying it from the seat allocation map on the Virgin website, which you can access through the booking code on your ticket.
You can also use this
tool on the V-Flyer site to see what aircraft you are likely to be on, based on the planes that have flown that route recently.
Hope this helps.
Nick