How much vacation you have is how much the company is required to pay out as earned income when you separate from the org. Anything else is generosity that can be rescinded without notice or recourse.

Edit: u/babycake's comment says it better: https://news.ycombinator.com/item?id=29879742

Yes, this is extremely easily solved. We have unlimited holiday, but you must take off at least 28 days (the statutory minimum in the UK).