Lots of people blame VANOS for just about everything. I dont think this is one of those times,though,The problem you discribe is almost always the result of a LAZY or DIRTY idle control valve and/or small vacuum leaks in the intake system due to crack or broken air hoses.
The car also need a basic tune-up. Any time you have a driverbility problem on a mature BMW, you first have to ask how many miles are on basic tune-up parts.
A BMW should have new spark plugs,air filter and fuel filter every 60,000 miles.