De or het vlaams-nationalist?

The correct Dutch article is:

de vlaams-nationalist

Rule Words ending in -ist: almost always de. See the rule
Meaning (Dutch) (politiek) aanhanger van het Vlaams-nationalisme

How to use vlaams-nationalist

the vlaams-nationalistde vlaams-nationalist
this vlaams-nationalistdeze vlaams-nationalist
that vlaams-nationalistdie vlaams-nationalist
our vlaams-nationalistonze vlaams-nationalist
your vlaams-nationalistjouw vlaams-nationalist
every vlaams-nationalistelke vlaams-nationalist
which vlaams-nationalist?welke vlaams-nationalist?
a big vlaams-nationalist / the big vlaams-nationalisteen grote vlaams-nationalist, de grote vlaams-nationalist

Meaning, plural, diminutive, pronunciation and translations: WikiWoordenboek (Dutch Wiktionary) (CC BY-SA 4.0). Example sentences: Tatoeba (CC BY 2.0 FR).