Almost all new starters at Virgin have been taken on fixed term contracts for the last few years. I don't want to say anything for definite because obviously things can and do change, but so far I've not known anyone who's wanted to stay and who's been up to the job being forced out at the end of a contract. Even though you're fixed term, you're eligable to apply for any VT internal vacancy, so you can apply for any permanent positions that come up, or your own contract may be made permanent.
I started at Virgin 2 years ago as part of a group of 9 fixed term CSA's (all on-board CSA's at Wolves). Of those 9, 3 are now permanent CSA's, 1 is a permanent ASM (rank above CSA for onboard staff), 1 is now a Train Manager, 1 is a permanent retail CSA at another station, 1 moved to platforms, gained a permanent contract, and then left to become a guard at another TOC (me), and 2 left of their own choice.
Like I said, no guarantees of anything, but keep your nose clean and your chances are pretty good - certainly a lot better than the hundreds of other people who will have applied for your job!
I've been there 6 months so far!