Hi!
Congratulations on the VS and the final stage DTC interview!!
I'd definitely say it is worth asking the vacation scheme firm first before withdrawing entirely, especially if the DTC firm is your preferred option. I was actually in a very similar position last year during one of my schemes - I was invited to a final-stage TC interview at another firm and there was no flexibility on the date, so I explained the situation to the VS firm and they allowed me to leave for a few hours to attend the interview.
Of course, I can't say for certain whether your VS firm will agree (particularly as it is on the first day of the scheme) or whether it could have any impact on conversion chances, but I do think it is worth at least asking the question rather than assuming the answer will be no. Firms are generally aware that candidates are often juggling multiple processes at once, and if you communicate professionally and give as much notice as possible, they may be more understanding than you expect!
I hope you are able to arrange something with the firms, and best of luck with the interview and/or scheme!
