You can get most out through the leaking head gasket :>)

Your supposed to remove one of the hoses that go from the radiator and
coolant pipe. Then it is a process of removing yet another hose( take your
choice), and another , and another. Each time jack the
Vanagon up in the front and back. Then side to side. You will soon
discover what good enough means in Vanagon coolant system maintinance,
because after all this, you will still have a quart or two still inside.

That means fill it all back up with  clean fresh water and go driving and
do it all over again.BTDT many times. Somebody has probably determined
exactly the bestest process for doing this, but it is still lots of work
to get most of the coolant out. Good  luck.