E-brakes should work ,even when left unusued. It sounds like the brake
cables are rusted.Is it ice, in the brakes freezing on the shoes and
drums? I suppose they can be unseized by working them back and forth . New
ones are better.BD has em,but you know that.There are little grease cups
on either end of the cable.I believe they are for sealing out water,
rather than relubing the cable.New grease.I would try squirting some  oil
on the cables, and allowing gravity to distribute it.Work them back and
forth ,then.

New cables are not expensive. Even for me. I think they can be replaced
with just removing the drum, and threading in ,new cables.Tighten them up
at the union, in the center of the van.Maybe remove the cables and lub
them up.

After releasing the parking brake, go underneith and see if the cables are
slack.They should be. Yank on them and see how frozen they are.It is
either money or work that will fix your Vanagon.Maybe both.