If you are at max vitality and gain x vitality off a card effect. does it still count as a gain even though you vitality does not change?

If an ability is looking at a future or pending gain, then it will see the full amount about to be gained (for example, Hades Cannon - if you Enhance with it, for the rest of the Enhance step any vitality gains you would receive are converted into damage).

If an ability is looking at the last time you gained vitality, then it will see the amount actually gained (for example, Death Cross Dancing - if you were at 18 out of 20 vitality and played Blink of an Eye, its full gain is +5 but if you then play an attack and Enhance with Death Cross Dancing it will see that your last gain was +2, for going from 18 to 20).

Dang, so that means that if I'm at 24 out of 27 vitality and use Admiration, "gaining" 20 vitality, then use Single Wing Slam's E (This attack gets +X damage. X equals the amount of vitality you gained the last time you gained vitality this turn), I'd only get a damage boost of +3? If so, that really messes up my strategy once Hades Cannon rotates.

correct, you'd only get +3 from the SWS E.