This isn't usual from my experience. I got an immediate email confirmation and then other emails before departure (i.e reminders to fill out ESTAs.......) You can book your plane seats 90 days before you fly and I also got a reminder for this. Do you have a booking reference from your original confirmation invoice? If so try to log onto the Virgin Holidays website and see your details.... Good luck and have a lovely holiday!
You won't get a reminder to book your seat but you can pre book 90 days before the day you travel. You just need to go to manage your booking on VA website & enter your booking reference number & name. If your booking is through Virgin Holidays, I think you have to go through them to book your seats.
You usually pay the balance for holidays 8 weeks or so before travel (how long is it til you go?).
If I were you, I'd get in touch if you've heard nothing from them about your initial payment though.