OK. Let me try
>Not Really.
>Where's RVA? Position of the aircraft or vicinity of the airport?
position of aircraft and intended path from current position to final approach path.
>And why is that 'the A/C must be below 2000ft before it can be cleared for >a visual approach.'
Only applies if ceiling is 2000ft.
>Suppose an aircraft is 40 nm from the airport, her position's RVA is 6000 ft, >and the airport's RVA is 2500 ft, ceiling (of the airport) is 3000 ft, can I >initiate vectoring the aircraft for visual approach?
Depends. In your example, at some point the RVA will have to step down from 6000ft to 2500ft. Therefore you would not be able to clear the aircraft below 6000ft until that point. Once the aircraft is inside the 2500ft RVA and below 3000ft (with a 3000ft ceiling) it can be given a clearance for a visual approach.