Note: Some of you, if not, all of you may know this, but I haven't heard or seen anything about this so I'm going to inform those out there who may not know. If you know, please keep your negative comments to yourself; thank you.
Everyone remembers when Harkon bit them and made them into a Vampire Lord. Well, people were more than happy with the new ability to feed on people while they're awake, but some wanted more. To feed on people as a transformed Vampire Lord like Lord Harkon did to them. Well, I have fount the answer:
When you have transformed into a Vampire Lord, go to any of the Holds, then tranform into a Vampire Lord. Once you get there, descend to the ground and fight with your claws constantly, and attack everyone you see. You will eventually see a character animation very similar to the one with Lord Harkon biting the Dragonborn to make him/her a Vampire Lord.
If you didn't know this, I hope you like it. Enjoy feeding upon humans as a Vampire Lord.