Who said anything about "sparsely distributed water vapor?"
My mistake. You said "you could find huge amounts of water ice around practically any star in the galaxy" after commenting on comets, moons, etc. and I took that to mean you were commenting on a more nebulous kind of phenomenon.
And the environment of an outer moon or asteroid is probably a lot less hostile than an inhabited planet surface -- no harsh weather, no dangerous animals, little solar radiation hazard, and again, much less gravity to fight. There's also the matter of distance.
Here I have to disagree. Harsh weather and hostile environments on Earth as compared to the moons and comets of space? I don't see too much clamor for beachfront property on, say, Europa.
Besides, why build an entire mining operation from scratch (or several of them) on the frozen surface of a comet or moon when you could make use of tech and labor from an already-inhabited world (one which has a relatively mild and hospitable environment) with a decent industrial infrastructure, but is technologically inferior? Particularly when that labor force is renewable and can be used as both food and fodder for future imperialistic operations?
If you were to siphon water on Earth, and didn't want to do all the labor yourself, and wanted to use your operation as a base of operations for an imperialistic agenda, where would would you go? To exploit an industrial center or to Antarctica?
It would take far less energy and effort for the Visitors to mine the outer moons and comets of their own star system -- which, again, should provide thousands
of times as much water as a single inhabited planet would ever need -- than it would take to travel across light-years to another star system. There's no sensible reason why any species would ever need to leave its own system for a resource as basic as water, because water ice is going to be one of the most abundant geological constituents of any planetary system in the galaxy.
And even if, for some reason, their water demands were so great that they needed more than one star system could provide, there are still at least five star systems
closer to the Visitors' home system of Sirius than Sol is.
Although of course Sirius is only 300 million years old and couldn't possibly support advanced life anyway, unless they weren't native to that system and their world is terraformed to begin with. In which case their technology would be so advanced that they wouldn't need anything as primitive and inefficient as human labor to help them acquire water.
Sure, there are other options for water, but even so, do they have a ready supply of food? Labor? Industry?
The point is, while I agree there could be better options for the Visitors if all they wanted was just water, it isn't "scientifically ignorant" to suggest that they might come here for their water, food and imperialistic needs -- all of which are stated in the original V mini-series and in KJ's sequel book.
Besides, isn't science fiction inherently about what is possible
, more than it is about what is probable
? And, in truth, while it's highly improbable that an alien force will arrive a week from next Tuesday to subjugate the planet, take the water and use us for food, it is possible