There are three things that will make a valve break next to the keepers:

1. Improper fit of the keeper on the valve or keeper not installed correctly on
the valve.

2. Fatigue. Only happens with very old parts.

3. During racing.

Unless you have been racing your vanagon :) # 3 is unlikely. Both 1 & 2 point in
the same direction: the mechanic.

If you have taken the head off you may find other things. Did the valve heads
hit the pistons? This is practally unknown with a gear driven cam. Does the
valve stem fit OK in the valve guide?