Your 8% refers to money you receive in May or June called 'vakantiegeld' which is 8% of your gross yearly income. Your 'toeslag vakantiedagen' or 'vakantiedagentoeslag' is the money the employer puts in a little piggy bank to pay your paid leave with.

So, I gather that, as you get more holidays when you have been working in a company for many years, essentially it means that the percentage of 'vakantiedagentoeslag' goes up, thus there being more money in the piggy bank to pay for more holidays for you.