Why do you need a vacuum relief valve for a water heater?
Or are you asking about a Temperature and pressure relief valve?
If so it is rated by the BTU output of the heater and the outlet hole size of the heater. Most have a 3/4" but some big heaters with High BTU's have a 1" hole.
Always use a T&P relief that meets or exceeds the max output of the heater.
so I looked it up. That could come in handy. Sometimes when we shut off a multi unit building, the individual WH's siphon out. No access no control. But do they have a tendency to leak?
Not very common, and only when the water gets shut off and drained, so that they actually open up. But they can and occasionally do stick once they are opened.