The way I read it, if a unit isn't disrupted, then it doesn't matter whether or not it has previously been reduced when it takes a hit; it just becomes disrupted. If you want to kill a reduced, un-disrupted unit, you have to hit it twice: once to disrupt it again, and once more to kill it.
I puzzled over that exact question for a few minutes last night myself. The quote above is the conclusion I came to as well. So really it takes 3 hits during a shot to kill a unit completely...
Fun game! We need to get cracking on a Vassal mod...