So, the real question is if the target audience is against using actual cow's milk, don't they have a heavy correlation to the anti-gmo crowd?
To answer your correlation question, probably yes -- but vegans' attitudes toward "tampering with nature" vary widely from the view that it's the worst possible thing humanity can do, on through to the view that it's the best possible thing humanity can do (e.g., David Pearce), with a broad spectrum in between. (I don't have an ideal example of the former group in mind, but I could mention Sea Shepherd's Paul Watson, who advocates deep ecology and opposes GMOs.)
I think we will see sharper divisions between environmentalist vegans (especially advocates of deep ecology) and animal rights/animal welfare vegans over time. I don't know how the population breaks down in the west today, or how many people have even had to think about it that much.
That said, looking at it now (and especially once synthetic meats come online, out of cost efficiency if nothing else), I think our children might look back on the practice of animal slaughter as being pretty horrifically barbaric.
Where are there humans going thirsty? Honest question.
So the consideration that "people are going thirsty" should have zero effect on how Californians deal with their problem. It serves nothing more than as an emotional device to make people feel bad.
I don't know enough about the water system and the patterns of shortages to understand the prevalence of direct competition between sets of users. Of course a weird thing is that municipalities may have locked in particular sources, so there may not be anyone in competition with us at all (for example, we in San Francisco have near-exclusive access to Hetch Hetchy, so water demand from farmers elsewhere in the state isn't likely to threaten our municipal water supply, at least not very directly).
I guess I need to learn more about the extent to which different users actually compete with one another. Clearly livestock farmers in the Central Valley are in competition with crop farmers there for the same water resources, but their competition with city dwellers for sources of what might have been human drinking water is a thornier question.
California could go the desalination route, but so far that progress is painfully slow and the methods being selected may result in significant harm to local marine ecosystems: http://www.reuters.com/article/2015/04/15/us-usa-desalinatio....
Something like this would be much better: http://waterfx.co/. Unfortunately, last update in their blog is a year ago...
I also did a Google web search for the same terms and found, among other things, several articles by pro-GMO vegans complaining that it was common for other vegans to be anti-GMO (based on their personal experience, not studies). The former felt that widespread vegan opposition to GMOs was unfortunate because it would make it less appealing for people to go vegan (for example because the range of available foods would appear more restricted), or because it could make vegans appear indifferent or hostile to scientific progress, among other concerns.