What methods are there for storing electrical potential energy in water vapor?
A "bottle capacitor" is called a Leyden Jar.   The original one was full of wine, and held in a damp human hand.  The wine is one capacitor-plate, the hand is the other.

Thunderstorms themselves are storing charge on their raindrops.  Not on vapor.

That way, if all the raindrops get charged the same, they can't zoom rapidly outwards from repulsion (because they're large heavy objects, not tiny molecules.)    

And, if they did start repelling outwards, well, the region of falling rain might be a half-mile wide.  The drops won't be repelled apart significantly, compared to a half mile.

Inside a tiny bottle, any charged droplets (or vapors) will repel outwards within about a second or two.  They end up stuck against the glass.  You'll have a charged bottle surface, with neutral gas inside.

What solves this problem?   CAPACITORS!   If you put any region of negative charge right next to a region of positive charge, then insert a barrier so they cannot attract together and cancel  ...then you can store some electrical energy (stored in the barrier, stored in the strong e-fields between the two.)

Thunderstorms do this by having positive charge on small droplets, and negative charge on larger drops, then letting the positive droplets lag behind as they're falling.  Gravity and air friction acts as the "virtual barrier" between the two regions of falling, opposite-charged drops.