Yes indeed, they are separatist. But this is not the issue they get votes for. The party became big on a plain xenophobic and racist agenda.
Flanders has other natonalist party's, and the demand for more own power lives in all Flemish political formations
Belgium is a federal state, it took 40 years to build what we have now. There is a Flemish government, and a Walloon, and a Brussels.
Every region has autonomy in many area's and this will expand in the future as every politcal party in Belgium has demands in that area.
The cordon:  this excludes 'Vlaams Belang' from executive polital power. This works on every level, regional and local. This means 80% of the voters support partys that refuses to cooperate with the 'Vlaams Belang'
This cordon was agreed because of the xenophoob and racist agenda of the 'Vl.B'. Separatisme has nothing to do with it.
As for closer ties with The Netherlands: nobody there even want to talk with the 'Vl.B' Here in Flanders this is not an issue at all, even not for the 'Vl.B)

The struggle of man against tyranny is the struggle of memory against forgetting.(Kundera)

Maybe a diary on the Flemish political spectrum (sort of like this one) would be instructive and help me clear my misunderstandings. Specifically, I was under the impression that the Vlaams Belang and its predecessor the Vlaams Block did not invent separatism in Flanders and are rather an extreme outgrowth of mainstream nationalism which has existed for decades.

Those whom the Gods wish to destroy They first make mad. — Euripides