Why couldn't Tara be healed with vampire blood? If a person's leg is cut off can it be regrown with vamp blood?

Because technically Tara was already dead. When Debbie shot at sookie in the season finale of season 4, Tara saved her sejak pushing her out the way, she was shot in the back of the head. Pam pointed it out in the beginning of season 5. So that's why Tara is a vampire now.

BECAUSE SHE IS DEAD VAMPIRE BLOOD DOESNT BRING PEOPLE BACK TO LIFE IT BRINGS THE BACK DEAD LIKE A VAMPIRE