Thorsten,
I'm not very good at this,
but had a similar problem that was solved by something like:
amenity=vending_machine & vending~'.*parcel.*' [0x3006 resolution 18]

I believe vending~'.*parcel.*' means 
the vending tag contains the word parcel,
but I think the rule has to start with a regular expression like
"amenity=vending_machine"

Hope I'm not totally off base here

BennieD