I have 2 water heaters. Can I leave one with the water heater dial set to PILOT & the temperature set to VACATION, and only use one heater?

It all depends on how the water heaters are piped. Series, or parallel? If in series, you would need to keep the second heater in series in use and shut the first down as you described, or just shut the gas off completely. If parallel, you would need isolation valves to shut the water off to the unit you don't want to use and shut the gas off to that one. More info and we can help you out.