Vampirism is acquired through a questline after being bitten by special bloodfiend mobs, a vampire player at a ritual site in the last alliance zones, or you can simply buy it from the crown store (I'm not certain if you get the quest if you do it through the crown store).

Werewolf is similarly acquired in a questline by being bitten by special werewolf mobs in the final alliance zones, being bitten by a werewolf player at a ritual site, or through the crown store.

Being bitten infects you, which you can choose to ignore, get cured by a priest in the town in the final alliance zone, or do the quest to become a vampire or werewolf.

If you become a vampire or werewolf, you can get cured. Make sure you remove your skill points beforehand, otherwise the skill points stay in the skill line and you'll have to be re-transformed to get access to them. If you get cured and later re-transformed, you dont have to re-level the skill line.

You cannot be both a vampire and a werewolf.