According to Van Dale a vliegveld is a site for the landing and departure of airplanes and a luchthaven is a big vliegveld. http://www.vandale.nl/opzoeken?pattern=vliegveld&lang=nn
But according to the NL wikipedia, the term vliegveld is the general term for airport. And you can divide it in luchthavens (for civil aviation) and vliegbases (for military aviation).
However, both terms, vliegveld and luchthaven are used interchangebly.