I had this exact problem and was something I was really stressed about, but it ended up being zero problem.
However, the important part is, you likely have to tell them it's for a vacation scheme. Check the terms of your contract, but it likely says (it had in mine) that you can't take any paid employment elsewhere while still employed at that company. Technically, a VS puts you in breach of that and you could get in trouble.
What I did (and I would advise doing) is emailing your line manager as early as you can to request the time off, making it clear it's for a VS. You don't have to go into too much detail but if you just say it's an opportunity you'd really appreciate etc, I'm sure it will be fine. If will depend from employer to employer but if you're in a legal environment, they will know obtaining a VS/TC is going to be the ultimate aim.
Just be transparent about it and that's the best option