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."

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.

To paraphrase Eliezer, ceteris paribus and without anything unknown at work, water vapor is a greenhouse gas and ought to make the Earth hotter. Also, ceteris paribus and without anything unknown at work, a hotter Earth ought to lead to more water vapor in the air.

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?

The answer is "no," and I think the mistake here is two-fold. First, rough reasoning gets exponentially rougher as you travel along a chain of deduction. Second, we can't ignore the fact that the Earth's climate is a complicated system which has been around for a long time. The normal assumption should be that if you push on such a system, then it will probably push back at you.