The vorspiel is the opposite thing of an after-party (naschpiel/nasch). A gathering of the people going out (or to a party (usually not too many people)) getting suitably drunk together. A vors is usually held at a private place, like hiding in your parents bathroom with your parents stolen liqour at the age of 14, in a roundabout by the gas station with a couple of ciders at 16, in a comfortable basement with some beer and punch at 18, in your friends new condo with a short black dress and champagne at 20 - I guess. A vorspiel can be anything. Elin and I usually have make-up vorspiels, we show up raggedy and play loud, brilliant music while getting dressed, sipping some battery or red bull and putting make-up on. It's great, really.
Vorspiel is so common in Norway because going out is so incredibly expensive. Alcohol in general is - but especially out on the town, so getting drunk before you go to the actual party or club or concert has proven economic and praises the works of Darwin. There's almost always some chick too drunk to ever leave the vorspiel at all, which keeps the weakest off the market - unless their friends are significantly strong and/or determined, obviously.
This is an important factor to scandinavian nightlife and so a major thing to most Norwegians, which is why I thought you should know! But if you still don't understand, I'll give it to you in pictures:
Naschpiel: (After-Party, look - it's morning already!)
Vorspiel: (Pre-Party, though this is actually THE party, but you get my point)
See the difference?