"you dont have to swap the belt. if you did, then it wouldnt truly be interchangeable as you'd have to swap the crank cog and cam gears also."
one has nothing to do with the other. as long as the teeth are the same on the crank and cam gears it will still work. But the diff number of teeth on the water pump make for a diff length belt. This is why you have to use the belt that matches the pump you are using it on unless of course you are using a B18c pump on a B16 motor then you will need to use the shorter B16 belt.
However I do agree that any vtec pump will work as long as you have the vtec belt that matches your block
I also have heard that you can use the ls water pump if you want you just have to use the coresponding ls belt to go with it. However the ls pump is not good for the high rpm vtecs turn. This info comes from ls/vtec swaps.