"Your scenario isn't really valid. What happens is that individual molecules achieve escape velocity and fly off into interplanetary space."

Venus escape velocity is 10.3 km/s
Escape velocity from the Sun at Venus distance is 49.5 km/s.

For an object to get escape velocity from Venus does not mean it escapes Venus's orbit around the sun. The vector of the velocity is an important and determining element.

We shouldn't expect most molecules of H20 to achieve vectors that are calculated at JPL, though there may be a tendency for the H20 molecules leave Venus in some particular direction.

There could be a process that amounts to the molecules "solar sailing" or charged particles of solar wind are providing the solar escape velocity needed. But without some kind process like this, the H20 molecules would have tendency to remain near and/or crossing Venus' orbital path.
Though there is a lot space involved around Venus orbit- if you added one molecule of H2O to each cubic inch of that space, oceans could perhaps "disappear" into it.

But it one should be able to detect where a Venus ocean went to, assuming there was one at one time and it left.

Though I think it's possible that a large chunk of Venus' ocean is still in it's atmosphere- and simply has not been completely measured yet.