The content will be available early on in the Skyrim game, but the real meat won't begin until level 10. Hours in, you'll face a choice - do you want to be a Vampire Lord or do you want to join the Dawnguard, the vampire hunters? You'll hear about the new content through Skyrim's guards, who tell you the Dawnguard are recruiting.
In Dawnguard, you can become a Vampire Lord.