What happens to spent perks if you cure yourself from being a vampire lord?

Solution 1:

Once Vampire Lord/Werewolf perk points are used, they're set permanently. If you cure yourself and then get re-infected you'll retain all of the perks you originally unlocked. As the wikia article describes it:

Perk points allocated to the tree cannot be undone, although the vampirism can be cured through Falion's ritual. Requiring a filled Black Soul Gem, the affliction is reversed but all perks remain.

Also, it's not immediately clear what you mean by "to be spent elsewhere", but perk points for VL/Werewolf are drawn from their own pools. You can't spend them on the normal perk trees or on each other's trees.