I’m a Werewolf. How to become a Vampire

The upper Echelon of The Companions (Fighter's Guild), called The Circle, is composed of Werewolves. As you advance through the Companions quest line, you will eventually be inducted into The Circle, whereupon they will perform a ritual to turn you into a werewolf.

(You need to become a werewolf to continue the Companions questline, but you aren't forced to complete the quest (and become a werewolf) if you don't want to).

And by extension, how do I cure lycanthropy, that is, if there even is a cure?

Fortunately, you are also introduced to the cure when you are doing the Companions quest line. You'll be asked to kill some Glenmoril Witches by Kodlak, and make sure to hang onto their heads. The questline will tell you exactly how to cure your Lycanthropy.

Source: Cthos

Make sure you keep the latest savegame before that contraction!

The uesp wiki states the following:

You can also cure your vampirism by becoming a werewolf through the Companions faction. However, once you contract lycanthropy, you gain a 100% resistance to disease and cannot be re-infected with Sanguinare Vampiris by normal means while a werewolf.

and

The first time you transform using Beast Form, you will immediately be cured of all diseases, including any stage of vampirism. This means you cannot be a vampire and a werewolf at the same time. The Beast Form power also automatically removes all of your equipment, and all summoned creatures (including dead thralls) will immediately die.

but

Accepting Lord Harkon's gift of vampirism during the Awakening quest or asking Serana to make you a vampire (either during the Chasing Echoes quest or after the end of the Dawnguard questline) will also cure you of lycanthropy.

So there are about three possibilities:

• You will be cured of vampirism when turning into beast form, without ever becoming an actual vampire
• Volkihar vampirism is indeed stronger than lycanthropy and you are actually no longer a werewolf
• By a bug similar to

If a non-werewolf contracts Sanguinare Vampiris but has not become a vampire yet, becoming a werewolf will not cure vampirism. This will progress to the player being both vampire and werewolf at the same time. Since the werewolf feeding ability overrides that of the vampire, the player cannot feed as a vampire and there will be no way to prevent progressing to stage four vampirism.

you have successfully managed to become a Wermpire / Vampilf...