Here's my understanding on the water pumps. They are geared differently to accomadate the different rpm limits of the engines. An LS will not go beyond 7000 rpm, at least not in normal operation it's going to survive, so the water pump is geared to work most efficently in that range. A VTEC water pump can see speeds in excess of 8000 rpm and must be slowed down compared to an LS pump to work efficently. If you overspin the pump, it will cavitate or churn rather than pump coolant. A change in the diameter of the gear on the water pump also requires a different length timing belt. The tensioner might have enough range of motion to work with either but from what I've read, use the timing belt that goes with your water pump.

I don't imagine having an LS pump and VTEC rev's would be a huge problem unless you spend a lot of time in the upper rpm band but if I was building the engine (and I've never build an LS/VTEC so I can't comment from experience, just research) I would go with a GS-R water pump and timing belt. Knock sensor I can say is nessicary or you will have a code. I've read about tappign the alternator bracket and I think the knock sensor hard spot on the block is even present in some LS blocks, just not drilled and tapped.