The furnace vent and especially the water heater vent are probably natural draft appliances meaning the combustion product(carbon monoxide, etc.) need to naturally vent out of your living area so moving the vents is probably only going to be possible if you move the appliances. Your local codes officials can tell you what the clearance requirements are for vent pipe and you can reduce them by going to double and triple wall pipe. If your venting runs through an attic and that attic has enough clearance you may want to consider moving both appliances into the attic and reclaiming all the floor space