In order to get the rear seat/bed to be flat, first pull the release handle
in the back of the seatback to unlock it.  Then tilt the seatback slightly
forward to keep it from re-engaging the latch.   Next, pull forward and up
on the seat bench.  This also pulls the seatback into the horizontal
position.  (This sounds much more difficult than it really is.)

I suspect you were simply pushing the seatback towards the horizontal
position, which you have found will not work properly.

Also, if you have head rests on the rear seat, they must be removed prior to
making your bed.  They either have a quick disconnect lever (in later
years), or else you need to remove a circlip (hidden just under the fabric).
The headrests can be quickly reinstalled when desired.

Welcome to the wonderful world of Vanagons!

Budd Premack
86 Syncro, 73 Super
Minneapolis, MN (Land of Sky-Blue Waters)