Is your van fully loaded? If not I'd maybe run the pressure at VWs minimum recommended pressure for the load being carried. Both static and sliding friction between a tyre and smooth ice is completely independent of its surface area, at a microtextre level.
However the tyres ability to deform and wrap around larger chunks of ice at a macro level, may benefit from a lower tyre pressure. This would help the tyre to put down a tractive force where your driving with these types of conditions.