ok. looks like you have the normal dim-dip system stops you driving round with no headlights on. to check this
1. With the engine running, switch on the sidelights - you hould have sidelights on and dipped beam on at reduced power
2. Leave the lights switched to sidelights and turn off the engine - sidelights (parking lights) should remain on and the dipped beam should have extinuished.
i'm not 100% sure but i think the lights know your engine is on by been connected somehow to the oil pressure switch
i suppose to just have sidelights only you could wire in another swith to them both but it sounds like your van is doing what its supposed to do