Provocatively worded... but intriguing from an engineering POV: if it is too late to limit CO2 levels and return to pre-industrial levels, what levers do we have to control water vapor levels?

Nothing. Water vapor is always in equilibrium with oceans on the timescale of weeks. In other words, any excessive water vapor you manage to put into the atmosphere will simply rain down.

http://www.realclimate.org/index.php/archives/2005/04/water-...

Not so fast, please. The very link you provide refers to the Pinatubo eruption that caused a significant multi-year cooling. It is well-known that cloud albedo changes in response to the addition of aerosols, which ultimately changes cloud water content (rapidly adjusting equilibrium with changes in global sea-surface temperature). Thus, there is at least one lever of action that we know of.