Yes, 50-50 is the only way to go. The only way to completely drain the
system is to open both heater valves (front and rear) remove both
radiator hoses at the front, the connections at the engine, the lower
connection at the expansion bottle, and, last and the biggest PITA,
remove the pushrod shields and you will find drain plugs on each water
jacket.

You can skip the last step (which can take you all day!) and flush the
old coolant out of the engine by hooking everything back up, filling
with water, running the engine for a short time, then disconnecting
everything again.  You can also install a tee (Prestone makes one that
is available from any auto parts store) in one of the heater hoses after
draining, and hook up a hose to flush the system with water.

Since you can't get all the water out without removing those plugs on
the engine, add the proper amount of straight, undiluted coolant (50% of
the system capacity) and add water to fill the system.  Bleed, and top
up with more water.  After bleeding and a week of driving, if there is
any coolant drop top up with 50-50 mix and look for leaks!

 It is a time consuming process, which is why I now use the long life
orange stuff, Havoline Dexcool or Prestone Extended Life, so I only have
to do this every five years instead of two years.  Both are best for
Vanagons anyway, ethylene glycol based, no phosphates, and utilize a
totally new anticorrosion chemistry that doesn't break down to form
corrosive acids like the old stuff does, including VW's Autobahn
coolant.

It must be fall, the time when young men's hearts turn to changing their
antifreeze. . .or something like that!
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia
'65 MGB (Driven since 1969)
'74 MGB GT (Restoring)

Assisting on Restoration:
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Parts cars:
'68 & '73 MGB, '67 MGB GT