This thread does contain spoilers about Dawnguard DLC. Turn back if you don't want to know.

I intend to separately collect all of the perks for both Vampire Lord and Lycanthropy.

Here is my list of how I want to go about this:

1. Become a Vampire Lord, collect all perks, complete the Dawnguard quest line.
2. Become a Werewolf, collect all perks, complete the Companions quest line.
3. Become a Vampire Lord again.

What I am wondering is, will I need to get the vampire perks again, or will I have all the perks that I've previously earned when I become a vampire again?

You will retain all perks, both Vampire Lord and Werewolf, if you cure and subsequently reacquire the condition.

Note that while Vampire Lord condition can be cured and reacquired as many times as you wish, the Werewolf form can be reacquired only a single time.