Because damage remains marked on a creature until it's removed as the turn ends, nonlethal damage dealt to Angel of Vitality may become lethal if your life total drops below 25 during that turn.

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, Angel of Vitality's second ability will apply twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, planeswalkers, and/or battles at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will apply only once.

The middle ability of Angel of Vitality applies just once to each life-gaining event, whether it's 1 life from Dazzling Angel or 4 life from Apothecary Stomper.

If Angel of Vitality is dealt damage and your life total is increased above 25 before state-based actions are performed (most likely because Angel of Vitality is dealt combat damage at the same time that a creature you control with lifelink deals combat damage), Angel of Vitality's last ability applies before checking whether it's destroyed for having lethal damage marked on it.

If you control two Angels of Vitality and you would gain life, you gain that much life plus 2. A third Angel of Vitality has you gain that much life plus 3, and so on.

If you gain an amount of life "for each" of something or "equal to the number" of something, that life is gained as one event and the second ability of Angel of Vitality applies only once.

In a Two-Headed Giant game, life gained by your teammate won't cause Angel of Vitality's second ability to apply, even though it will cause your team's life total to increase.

In a Two-Headed Giant game, Angel of Vitality's last ability applies as long as your team's life total is 25 or more.

In formats where your life total doesn't start at 20, Angel of Vitality's last ability still applies if your life total is 25 or more.