Fighting Lord Harkon is the same regardless of whether you are a member of the vampires or the Dawnguard. If you are not prepared, Lord Harkon can be a difficult opponent to defeat. If you gave him Auriel's Bow, then you will be at a slight disadvantage.
It's possible to fight using Werewolf, the fact no human enemies fight outside of Harkon means you might die pretty easy from taking hits without being able to heal.
To answer the question directly, No you may not join the dawnguard if your a vamp already. But once you finish all the side mission and main quest and you still want to become a vamp Lord, by all means go right ahead.
Holding down the left bumper has the Vampire Lord flap its wings and fly-sprint, and the effect can be sustained for a relatively long time.
At lower levels, Werewolf is maybe a better choice, because it's easier to level up, and Vampire Lord won't have as many advantages. However overall, Vampire Lord is a complete power trip. At level 46+ you get an extra 250 health, 200 magicka and 100 stamina.
The flashback scene takes place 20 years prior to the main events of the game, making Drake 34 or 35 years old. Sully also complains in that game that Nate and Elena crashed his plane 4 years ago, obviously referring to the crash in Drake's Fortune.
Uncharted 4: A Thief's End - Age 40 It's completely possible that Nathan Drake may be a year or two younger, particularly if he meant he was in jail at age 15 and met Sully at age 14.
Nathan is around 31 years old in Uncharted 1, ScreenRant has reported. In Uncharted 3: Drake's Deception, Nathan and Sully are discussing that they met twenty years before the events in the third game, when Nathan tells Sully: “I should have known when I met you that I'd be in prison within a year!”