What kinds of possible evidence would you expect to see if such positive feedbacks can happen?

I'm not sure what you mean by "can happen," since in some sense lots of things "can happen."

Sorry for the ambiguity. I should have reflected your wording more closely and written "What kinds of possible evidence would you expect to see if the climate operates by positive feedback in this way?" Part of the purpose of the question was to determine what you meant when you chose that wording.

Anyway, it's not a full answer to your question, but the gold standard for substantiating the water vapor feedback hypothesis would be if the proponents of that hypothesis made specific interesting and accurate predictions about future events.

Since the effects are alleged to take place over decades, asking to see this evidence now is asking for impossible evidence.

I disagree, and perhaps an anlogy would help: All things being equal, cooler weather can be expected to lead to more snow cover. And all things being equal, more snow cover can be expected to result in cooler surface temperatures because of effects on the Earth's albedo. So should we worry that the next big volcano will trigger an ice age?

A priori, yes we should. However, we would be justified in decreasing our concern if either (1) additional theoretical consideration show that, in fact, according to our best theory, that loop probably wouldn't occur, or (2) despite our best theory, we've observed many big volcanoes erupt without setting off such loops.

Let us suppose that (2) is the case. Then this would decrease our confidence in our best climatological theory. However, if that same theory asserts that X will probably cause Y, where X is not very similar to something that we've observed in the past (so, not a big volcano eruption), then our best bet is still that Y will follow X, even though our theory blew it on the consequences of the volcano eruption. Our confidence in Y will go down, but it will exceed our confidence in ~Y. (Otherwise, the theory wouldn't be our "best".)

(To the best of my knowledge, our theories don't mispredict the consequences of volcanoes, though, for all I know, that could be only because volcanoes were part of the input data used in the theories' construction.)

The normal assumption should be that if you push on such a system, then it will probably push back at you.

This sounds like you want to construct a climate theory by taking an a priori first-principles theory and adding an ad hoc "push back" mechanism, according to which the current equilibrium is assumed to be more stable than the first principles would justify. It's fine to believe in such a mechanism, even if you can't justify it from first principles, provided that you have direct empirical evidence for it. In which case, great, add that evidence to the pile of all the other evidence that we use to justify beliefs about the climate, and let's see how it all adds up.