the classical method requires very specific timing and is difficult to accomplish. here i introduce a new method that is simpler and requires no such timing and is more robust.

serana must follow. you do not have to be a vampire lord or vampire at this point. use the well of aela's blood. wait for the load screen. there will be a 5 second window after loading where you can turn 180 and talk to serana, before beast form is added. the quest marker should still be pointing at the underforge. serana should be standing near the quest marker. request a vampire transformation from her. your werewolf body should go blue, similar to the way a human should. wait. once you wake up next to aela, your transformation should be complete, with both forms ready to deploy. this method can be used in conjunction with the old method - effectively doubling your success rate.

the real problem is the game itself, being rife with bugs:

first, sometimes the game will simply disallow vampire lord transformations. use
set dlc1vampirelorddisallow to 0

sometimes the game will allow you to become a vampire lord, but then will change its mind and revoke it.
set playerisvampire to 1
or if you use a custom race, the game will get confused
player.setrace nordracevampire
swap out the nord string for the appropriate corresponding string.