VIS just stands for Variable inlet system. The manifold is made up of a number of chambers with butterfly valves. The VIS motors control the opening and closing of these valves changing the volume of the plenum.
There are two VIS motors a Balance and a Power. These can often fail causing the car to burn more fuel and feel flat at certain revs.
The manifold itself has an internal arm that is connects each of the butterfly valves to the VIS motor. This arm attaches to the butterfly valves using ball joints that can wear overtime and become very slack and even, in some cases, disconnect from some of the valves completely . Obviously this will reduce the power of the car and sounds like a bag of spanner rattling.
You can pick up second hand manifolds on the cheap now and then. Worth fixing regardless of track use or not as ifs its knackered you will be seriously down on power
