Here in the UK, we can give vectors in any airspace, as long as the aircraft is receiving a radar control, radar advisory, or radar info service, the latter two being ouside Class A airspace. However we aren't likely to see any VFR in Class B in the UK. If an a/c cannot take the vector due to cloud(vfr only) or terrain, then it is up to the pilot to state this and the radar service will probably be cancelled.
[This message has been edited by 5milesbaby (edited 03 June 2001).]