If you do it for philosophical/religious etc. reasons...doh, it's a part of milk. Is it dairy? YES. Is casein "dairy"? YES.
Think of it, what makes dairy - dairy? The casein, the whey, the fat, the minerals, the enzymes, the lactoferrin etc.? Better yet, is there any other source of casein/whey proteins EXCEPT dairy? None that I know of (maybe a lab might work...btw, if it's made in a lab from anything but milk, it's not dairy)
OTOH, depending on your goals (strictly and truthfully respect the doctrine you're following OR see how/if you can get by a rule while theoretically respecting...), the best thing to do is to ask an authority in that certain doctrine, not people who don't even know what you're speaking about.
IF you're vegan, no IT IS NOT dairy. No really, do you see any milk in there? You really have to build muscle.
IF you're anything else but vegan, IT IS.