Dirk Hendrik Theodoor Vollenhoven (1892-1978) was Herman Dooyeweerd's brother-in-law and the other major reformational philosopher associated with the Wijsbegeerte der Wetsidee.
I will admit to having had little contact with Vollenhoven's thought over the decades, although my studies at the ICS could have brought me into contact with it if I had studied something other than political theory. In any event, I have added to my sidebar a link to the Vollenhoven Newsletter, the (sporadic?) publication of the D. H. Th. Vollenhoven Foundation in the Netherlands. Here also is a link to Glenn Friesen's Vollenhoven pages, for those interested.