This is a fairly common question at an interview stage, and so it is one to prepare for.
If you have got an interview though, it is reasonable they have already worked out that this is the case and by inviting you they clearly don’t care/worry about it that much.
It’s very common for people to not secure TCs from VS. Sometimes people are not matched with the right firms, sometimes they are just not ready and need some more time to develop their skills or motivations. It doesn’t mean they are a “bad” candidate - it just means the vac schemes at the time were not the right opportunity for them. But with applying to different firms and developing your skill set over time, the next TC opportunity could be the right one.