van Dijk is the commonest spelling in the Netherlands. Van Dijk with capital V in Belgium (after Napoleon). van Dijck is the original spelling of the dutch middle- and eastern- Brabant branch
and may stem from the original inhabitants of the famous Dyck watercastle in the old duchy of Gulick, now just across the border in Germany. The misspelling took place in the mid 18th century's church records. Around that period we also find van Deijck as a spelling of the same family. One branch of the family wrote the name as van Dyck or Van Dyck.
In Dutch the ij is a diphthong (pronounced somewhere between i and a)