Dutch law doesn't leave holiday pay to an employer's discretion. The Wet minimumloon en minimumvakantiebijslag sets a floor of 8% of your gross salary earned over the vakantiegeldjaar, the year running 1 June to 31 May rather than the calendar year, and almost every employer pays it as a single lump sum landing between 21 and 25 May.
That lump sum is not a tax-free bonus. It is ordinary Box 1 salary income, and payroll withholds tax on it using the bijzonder tarief, a special table based on your annualised salary level. Because it stacks on top of a regular month's pay rather than being spread across all 12 months, the effective withholding rate applied to it approximates your marginal rate, close to the top of your normal Box 1 band, which is why the May payslip's withholding percentage looks higher than usual even though nothing about your annual liability has actually changed.
If the 30% ruling applies to your employment, it doesn't stop at your regular salary. Vakantiegeld is calculated on the same combined gross income the ruling shelters, so the tax-free carve-out covers the holiday allowance exactly the way it covers everything else you're paid that year.
There is a narrow opt-out. Once your salary clears roughly 3x the statutory minimum wage, an employer and employee can agree in writing to accrue less vakantiegeld, or none, on the portion above that line. It has to be explicit in the contract though, most agreements, even at senior salary levels, still pay the standard 8% by default.