How should I handle the Dutch? I would like to somehow vassalize the netherlands.
if you move your capital to the lowlands the dutch don't revolt, or you can culture convert all the dutch provinces (moving your capital is much cheaper)
If you want to have the netherlands as a vassal your best bet would be to keep the area fractured into OPMS and hold one dutch province yourself. You will automatically be at war with the netherlands when they spawn out of that one province, and since it's only one province, you can 100% and vassalize them really fast (you need to be REALLY fast to make it before all the dutch OPMS join).
Alternatively if you full annex the netherlands they lose all their cores. (same as above but annex instead of vassalize)
Since you'll need diplo ideas for annexation cost reduction anyway, you should try to become emperor of the HRE.