Yes! Battling Pokemon still gives you the EVs and Vitamins still work in the usual way (10 only). The stats don't change there and then but they do "resolve" themselves when you put the Pokemon in the PC and take it out again.
Sort of! Level 100 Pokemon don't gain any EVs from battling. They can gain EVs from Vitamins though, and lose EVs with the special Berries, which will change the Pokemon's actual stats on the fly. This means you can technically get an Effort Ribbon for a level 100 Pokemon, but you must apply Vitamins across all stats. That's 51 Vitamins, if your level 100 Pokemon doesn't have any EVs yet.
Yes! Effort Values are now applied after every battle so your stats can change while you level up. And this continues at level 100. Vitamins, Wings and Berries apply the stat changes on the spot, too.