Go to "my account" and under the information that's under the box that tells you how much time you're paid up for on your current plan, there's a big button that says "cancel my membership." Press that and the rest should be self-explanitory.
I'm assuming it's a 2K sports game you're talking about. The difference is that "my career" is a dynasty mode while "my player" is more like an RPG mode where you take one player through their own career.
You get 11 perks that no other class can get, in addition to Vampirism's normal effects. You can cast blood magic, which is improved by a Ring of the Euridite.
You gain 50 extra health, mana and stamina while in Vampire Lord form.
Detect All Creatures. (Detects all creatures, even dwarven automatons.)
Mist Form. (Essentially the same as Become Ethereal, but with even more regen.)
Supernatural Reflexes. (Time slows down while you still move normal speed.)
Blood Healing. (Killing people by melee-ing them from vampire form returns life.)
Poison Talons. (Your melee attacks do 20 poison damage.)
Night Cloak. (You are surrounded by bats who attack nearby enemies.)
Vampiric Grip. (Pulls an enemy towards you and chokes them once they're near.)
Summon Gargoyle. (Has similar stats to an Atronoch and attacks nearby enemies.)
Corpse Curse. (Paralyzes target for a few seconds.)