According to the National Rail page on advance fares, you can only book Advance tickets with Virgin upto April 28th.
I don't think I will ever understand these things !
That page doesn't update as quickly as the Virgin site does.
Although you can book train tickets through anyone whether it's another TOC (I still go to gner.co.uk to book mine, whoever it redirects to - Virgin East Coast now) or a ticketing company (such as thetrainline, etc) you will always find the advance tickets (and sometimes the best deals) available from the TOC providing the actual service.
It takes a while for the cheap advance tickets to filter down to the other TOCs and general ticketing companies. This is usually around the time that the National Rail site says they will be available.
The latter is generally 84 days but the supplying TOCs may make the advance tickets available as early as 90 days. If you go by the National Rail site you may lose out on the best deals.
To get the best deals I used to check the websites every hour or so (when I was awake and near a computer) from a day or so before 90 days...