First, you need to enable the Vampirism mod to become a vampire.[1]https://teamlapen.github.io/Vampirism/. There are three ways to become a vampire. The first is to get attacked by a vampire and contract the affliction they carry, Sanguinus Vampiris. The second way is to right-click while holding one of their fangs to forcibly contract the disease. The third way is to be bitten by another player.
Once you have the disease, you must wait 1 hour (3 Minecraft days) before you will become a full-vampire. You can speed-up this process by blood trading: First, a non-vampire will drink blood from you and afterwards you have to drink much blood of the infected. Please visit the races to see, what race is unaffected by vampirism.[2]https://massivecraft.fandom.com/wiki/Vampires_Guide