Once Bitten, Twice Shy (25 player)

Can confirm all you need is 'Essence of the Blood Queen' to 'become a vampire'- once you have the debuff kill her and you get credit for being a vampire!

Shamans can solo Once Bitten, Twice Shy (25 player). The second half of this achievement is fairly simple. Just blast down Blood Queen Lana'thel and you'll get [spell=70867] at a very early stage in the fight. All of your offensive cooldowns should be up when this happens, and she'll be dead in seconds. At this stage in the game, you will not have to worry about the biting component of the debuff.

The problem for me came with the first half: defeating Lana'thel WITHOUT becoming a vampire. Spec into [spell=117013] and use [spell=2062] to start the fight. LET THE EARTH ELEMENTAL AGGRO HER AND DO NOT ATTACK LANA'THEL YOURSELF. This will cause her to prioritize you for Essence of the Blood Queen and either earn you another week of waiting on the achievement or death. Once she says "Know my hunger!" you are in the clear and can blast her down.

Can confirm the above post. The first (and much harder) part of this achievement - i.e. not being bitten - is soloable as a Night Elf Druid in 6.0.3. The detailed strat is as follows:

1. 
   
2. Make sure you have Force of Nature talented (you need your Treants)
3. 
   
4. It helps to create a timer via DBM (/dbm timer 15 OnceBitten) but it's not necessary. The important thing is that she bites around the 18 second mark.
5. 
   
6. Target the boss, then pull with Force of Nature. Start your timer if you have one. The boss will attack your Treant. Wait a couple of seconds and then summon another one (in case she kills the first). Move away and to the side.
7. 
   
8. At around 15 seconds, but it can be any time after you have summoned the second Treant, hit Shadow Meld and wait for the "Feel my hunger" or similar notification to signify she has bitten the Treant.
9. 
   
10. Kill her, as she won't use her bite attack again.
11. 
    
12. Profit!
13. 
    

That's it. It took me two weeks, a lot of tries, and quite a few deaths to figure this out.