basicly varible valve timing, cos the engines are chain driven vanos is like a bolt on unit full of gears which adjusts the cam timing, iirc it's just on the inlet cam? unless it has double vanos which is both, iirc it's got a solenoid on it which makes it ecu controlled. vanos is simply the bmw name for the unit, like honda call theres v-tec.

It's about £1100 for the unit from bmw, common fault is excess noise from the unit, iirc theres a gear that wears and nothing can be done, the other fault is it stays advanced or something and i believe you can change a seal or solenoid to get it back running on course, although i suspect special bmw tools are needed for this.

But i'm no bmw expert, hopfully alex will confirm/deny though.