Water vapour is a greenhouse gas, but it comes back down as rain quite quickly. It's also more or less in (dynamical) equilibrium, because there is always a huge source of atmospheric water in the form of evaporation from the oceans' surface. The way to meaningfully increase atmospheric water content is to increase atmospheric temperature, which methane contributes to, but that's a second order effect.

I suspect that's not so much worrying as there are plenty of vapour sources on Earth (like the vast oceans)

More water -> less evaporation (but the partial pressure will remain the same more or less)