Personally, I would go on your holiday as planned. When I left PN for my new employer, I was called by HR to arrange an interview and I made it quite clear that I would be on holidays at their requested time, and would they mind doing a phone interview? They had no problem with that, the interviewers even got a bit of a chuckle after I'd explained that I was at SeaWorld and to ignore the screaming in the background - I wasn't killing someone.
I had to Google what that VTS testing thing was, as I've never done it, but have worked with people who have (and some of them I wouldn't trust to drive anything bigger than a go-kart, yet alone a coal train!). Given that they haven't called you to set a date for the VTS testing yet, should they do so there is probably more than a fortnight in which you can arrange to do the testing, that is to say, either before or after your holiday.
IF you make it to an interview stage, you can expect the standard questions, why do you want to work for QR, etc. But also job-specific questions, Have you done shiftwork before? If yes, how do you handle your fatigue? Tell us about a time something didn't go right for you? Use the SAO model to frame your response, describe the situation, your actions and then the outcome. IF it didn't go well, you may be quizzed on why, or what you learnt from it. Don't be afraid to admit you've made an error somewhere, so long as you can describe you learnt from it. They might also show you a printout from the Driver's Route Knowledge Diagram and say "You're going to get changed out by a new crew [here] , what issues can you see with that? Consider things like is it on an embankment, is it on a curve (bad sighting of oncoming trains), could we perhaps move it a little further down to [here] and explain why you think that might be safer.
Finally, and this applies to any operator, be bloody sure this is what you want to do for a job. There is shiftwork, then there is railway
shiftwork. You won't have a social life, and it will severely impact on your family too through lack of opportunities for social gatherings, kids need to be quiet because you'll be sleeping, etc. In saying that, I have done it for over a decade and wouldn't do anything else, but I have a good roster now.