We can easily manipulate the level of water vapor in the atmosphere; it's just not economical to do so. Just because something isn't worth being done doesn't mean it can't be done.
If it's unclear, I responded to the above because my impression was that you're casting doubt on our ability to terraform Mars, based on our not having resolved the global warming issue for Earth. If that's the case, then even leaving aside the obvious logical fallacy your statement is wrong on its face. We know how to terraform Mars right now if we want to, it just requires the investment of resources to get it done.
Adding water vapor is a relatively straightforward exercise that could be done on Mars or on Earth if the reason and the will existed. Removing water vapor would be more dicey, but there's certainly no need to do that in order to terraform Mars...
We certainly can, from the technical standpoint, do some terraforming of Mars. No question of that. Whether we have the economic ability and political will to see it through is in doubt. Can you imagine terraforming being done on an annual budget basis with Congress apparently unable to dependably sustain the continuity of any program that lasts longer than a couple of years? Not to mention working with Europe, China, Russia, Japan, India, and whoever on any rational project at all.