Just wondering what happens in following situation.

1. I become a vampire lord.
2. I max out my vampire lord tree.
3. I start companion quest and become a werewolf, thus curing the vamperism since they are mutually exclusive.
4. I max out werewolf tree
5. I become a vampire lord again

Would I retain all the vampire lord perks I have unlocked? What if from this point switch back to werewolf, would I regain all my werewolf perks?

thanks.

All the perks should remain but don't go purely off my word xD

You can become a Vampire Lord again by speaking either to Harkon or Serana.

just be very careful because you can only change back to werewolf once more after you have been cured.

I believe you retain the perks in the respective trees, so that if you return to being a VL or a WW you automatically still have those perks.