I have a diesel Rabbit pickup.  It has almost the same setup as a Vanagon.
In the truck, the injector pump would not pull fuel from the tank when it was
very cold.  I installed a small electric pump between the tank and the injector
pump to help it out.  It only needs to be a 2-3 gallons per minute size.

I also use a cetane booster additive in the fuel when it is really cold too.
It seems to help too.

You didn't mention glow plugs but I assume they are all good?

Tom
Illinois