The clutch is designed to slip when the temperatures are "normal", and only engage fully when required for additional cooling. Under normal conditions you would not have the drag of fan taking power from the engine, and it should therefore feel more "zippy" and less noisy. It sounds to me like your VX fan is working properly, whereas the Trooper fan was either malfunctioning, or it's design is not compatible with the VX. You should be able to tell the difference in the amount of force required to turn the fan by hand. If it doesn't slip at ambient temperatures, it's not working correctly. Testing under "hot" conditions would be more difficult. If you're no longer overheating, then you are golden.