What is the most vulnerable sign of the Zodiac?

It depends on what you mean by vulnerable. Water signs are the most emotionally vulnerable with Pisces being the most sensitive "outwardly" because the don't have a defensive "shell" like Scorpio and Cancer. Leos is also the most vulnerable fire sign because their egos get easily bruised by criticism etc, but they are good at hiding it with their proud exterior. Overall, Pisces is the most vulnerable to outside influences. I'm a Scorpio. :-)