No expert on it, but i believe the vacuum pressure relies mainly on your water pressure from the faucet, although a minimal increase might be apparent if the hose is shorter, or atleast laid out straight with no sudden bends
just unscrew the nut where the tubing attaches to the sink piece it and it will reveal basically an insert adaptor that goes into the tubing. pull the tubing off the insert adaptor, cut it and re-attach. very easy.
Shortening the tube will not do much for the amount of suction. The amount of siphon is related to the head pressure and the flow of water throught the plastic pump. The head pressure is simply determined by the hieght of the tank water compared to the height of the pump attached to the faucet. The higher the tank is over the sink, the stronger the siphon. The faster the water flow, the stronger the siphon as well.