Valve Water Sealing

Perhaps you should think of it as a water-tight seal, without regard to the water column height.

Re-read the requirements.

Relief valves on steam turbine condensers are commonly fitted with water seals so that any in-leakage will be water instead of air. The ones I'm familiar with have a small make-up connection, an overflow connection which maintains a minimal (couple of inches) water height, and a sight gage. The drawing here http://www.graham-mfg.com/usr/Product%20Manuals/IOM_ARV_0203.pdf shows how it's done. Of course, your seat area has to be horizontal (vertical flow) and the vacuum side has to be below the seal. This also assumes that water is preferred over air in your process.

< 34 feet (~10.4m), or the vacuum won't break.

The very purpose of the (motorized) valve is to break the vacuum when ever required. I wonder, if you have a water seal on the open (atmospheric) side of the valve, won't it prevent air from getting into system. You need air to get into system, when ever needed, to break the vacuum, right?

Water seals are provided in vacuum lines as to drain off the condensate and at the same time prevent air getting into system once the condensate has drained off.

You have not given us much information about what it is connected to etc But I think you have misunderstood what they require I think it is meant to be a seal/ trap to stop water from being sucked into the atmospheric open side of the valve when it is operated. This is done to stop water entering & contaminating the system. I would be contacting the manufacturer to clarify the meaning