The Vampire Willow lives in an alternate reality where The Master rose at The Harvest, making vampires a very powerful and dominant force in Sunnydale. A traveller from another dimension set events in motion which brought the Slayer to Sunnydale. Willow was killed by being impaled when The Slayer raided The Master's factory. Later, a spell was performed which summoned Willow to the true reality, before her death. Confused as to why vampires didn't rule, she ended up getting a group of vampires to storm the Bronze, trying to regain power. The Slayer ended her plans, and sent her back to her reality, which continued as normal and therefore killed her.
Willow was about 2 years old as a vampire. She was probably sired soon after The Harvest. Perhaps after The Master rose, and came to the Bronze, he sired many of the patrons for his army. In the alternate reality, Willow was thought of as strong, but that was only because no one was around to take on the vampires.