It sounds like you are doing a waterpump change out on  a 1.9.I feel your
pain.I did the same thing as you snapping bolt heads off.Just couldnt stop
with one head though.The advice here is to leave those bolts ALONE ! This
is too late for you,but some other grasshopper may see this.  I spent a
sunday at the yard getting one off another vanagon. A few weeks ago I
reattempted to fix my old housing , and I think it can be fixed.The keys
are heat and strong vice grips. I had studs remaining from the broken
bolts. I used mapp gas for many minutes on the stud and I got one out.The
remaining stud is still waiting for another day when I got nothing to do.If
there were 50 lower thermistat housings with 100 bolts, all would be
snapped off trying to get that stupid flange off(it doesnt need to come
off, and a new gasket for this comes with the water pump kit,so how would
you know?).