You do not need a visa if your nationality is on the list of visa free travelers for Aruba, Curacao and St. Maarten. This applies, for example, to Americans and citizens of the European Union. Also in a number of other cases you do not need a visa for the Caribbean Kingdom parts.
For Bonaire, St. Eustatius and Saba (the BES islands) you will need to apply for a Caribbean visa at the relevant embassy in addition to your MVV letter, even if your nationality is on the list of visa free travelers.