If you finish vampire quest and turn into vampire, can you go back

This can happen for several reasons:

• They already have the quest
• They already did the quest
• It is a quest line and they haven't advanced far enough yet
• It is a guild quest or a main quest

I have found (experience and logic) that these cause quests to not be 'shareable'. The share button will still be enabled but no notification of any kind is sent to the group or yourself. I have no reference for this however.

Both Vampires and Werewolves (PvE) will spawn in high-level areas, and their attacks have some chance of infecting you with the relevant disease. Once infected, you will have a new quest that you must complete to either become the monster you wish to be, or be cured of the disease.

You can not be infected by the Vampire disease if you are currently infected with the Werewolf one, and vice versa.

Vampirism Effects (more details)

You'll unlock a new skill line you can make use of, you'll also start to have these effects at various stages, until you feed on a human opponent (PC or NPC), which will reset it to "stage 0".

• Up to 30 mins:
  • 50% Fire Damage Weakness
• 30 to 60 mins:
  • -25% Health Regeneration
  • +50% Fire Damage Weakness
  • -20% Health Cost to Vampirism Skills
• 60 to 90 mins:
  • -50% Health Regeneration
  • +50% Fire Damage Weakness
  • -40% Health Cost to Vampirism Skills
• More than 90 mins:
  • -75% Health Regeneration
  • +50% Fire Damage Weakness
  • -60% Health Cost to Vampirism Skills

Werewolf Effects (more details)

You gain access to a skill, Werewolf Transformation that does pretty much what it says on the tin. While transformed, you'll have access to a new line of skills to unleash your inner puppy, and there are some ill effects of course -- such as a passive ability that increases damage taken from abilities in Fighters Guild skill line, +50% to all poison damage.

PvP Infection

Players can also infect other players with their condition.

For Werewolves, the skill Bloodmoon can be used once every seven days, acting as a rate-limit. For Vampires, the equivalent skill is called Blood Ritual. You have to use these skills at the ritual sites, so it's unlikely you could be turned against your will. Both require level 6 in the respective skill trees.