It's important to understand that the primary determinant of the amount of water vapour in the atmosphere is temperature, which means that other GHG gases impact the amount of water vapour and thus amplify their own effect. That's why water vapour doesn't get much consideration: it has a short cycle, it acts primarily as a feedback system, and have no feasible ways to directly increase or decrease it, unlike the other GHGs like CO2 and methane.
That's what the textbook says. The distinction between feedbacks and forcings is a largely arbitrary adoption to ease calculation and prediction of an extremely complex physical system. It's the same reason that water vapour and clouds are treated as entirely independent variables as well. In an ideal world with unlimited CPU power, we'd just simulate water vapour molecules and their energy would dictate their state and clouds would form naturally. We lack the CPU power so we've broken them apart to something we attempt to simulate. The problem is they ALL interact in a complicated way and verifying predictions on something with that many dials is hard. It makes plasma physics models look trivial, simplistic and basically fool proof. In practice, how many plasma physicists have great confidence in a generalized simulation of a brand new, never before tried plasma configuration? Until they can build a parallel real world machine and match variables, they have very little confidence. Without fail, they constantly need to revise the models after comparing to real world models. That is in a plasma, where we KNOW, with great certainty, the rules of interaction between all the particles in the box.
What does that have to do with the overall contribution of water vapour to the greenhouse effect? Well, it means that we shouldn't downplay or dismiss that water vapour, at any given moment, is responsible for trapping 60%+ of the radiation that is captured by the greenhouse effect. Simply classifying it as an integer feedback also includes a laundry list of assumptions about it's behaviour that we have no reason to believe is true. The most glaring of which being cloud formation and the inherent complexity of predicting it.
When we agree that our simulations of water vapour feedback(complete with clouds) has a lot of uncertainty I think that's important. Sure, in models it's a feedback and when we run a model, we can get good results with that feedback dial really small. That hardly seems to me a compelling argument that we've definitively shown it is bounded by that. If you try and model a plasma, and you can get it all working really well modelling only your electrons, you've done great work. You can not claim though that adding an equal number of ions is now easily predictable and you know the bounds of how it will alter your plasma. That's a bad joke, but a lot climate advocates too far away from the modelling layer seem to try and tell it straight faced as proven fact