There isn't a cheat to become a vampire since you can become a vampire without cheats,
it's already in the game.

    You have to fight a vampire and you should contract a disease after a while, though it
is much more difficult if you're wearing armor that resists disease, or if your race
resists disease. The disease should be called Porphyric Hemophilia. The disease takes
three game days to work through your system; you can either carry on like normal pereson
for three days or sleep in a bed/wait for 24 hours three times, after that, the next time
you lay down to sleep it takes hold, and vampirism manifests itself.
    You awake from a horrible dream, and you find that things have changed. Your skin is
paler and your eyes are a strange red color. More importantly, you're faster and stronger
and have a power at your disposal. 


There are 17 location in which you can find vampires:


- Fanacas Ayleid Ruins (North-NorthEast of Cheydinhal)

- Lipsand Tarn (North of Chorrol)

- Ninendava (East-NorthEast of the Main Quest dungeon Sancre Tor)

- Nornalhorst (East-SouthEast of Skingrad)

- Barren Cave (West of Cheydinhal)

- Bloodcrust Cavern (From the Mages Guild quest "Information at a Price")

- Crowhaven (From the Miscellaneous quest "Origins of the Grey Prince")

- Fort Carmala (South of Chorrol)

- Fort Hastrel (Well North-NorthWest of Kvatch)

- Fort Naso (Beyond the Reed River South-SouthEast of Cheydinhal)

- Fort Redman (On the west shore of the Lower Niben NorthWest of Fisherman's
Rock)

- Fort Redwater (From the Dark Brotherhood quest "A Matter of Honor")

- Fort Vlastarus (East of Skingrad)

- Gutted Mine (From the Daedric quest "Azura")

- The air below Jakben Imbel's house in Talos Plaza (From the Theives Guild 
quest "Boots of Springheel Jak")

- Memorial Cave (From "Order of Virtuous Blood")

- Redwater Slough (From the Miscellaneous quest "Vampire Cure")


    As a vampire, you have a particular taste for human blood. Your abilities as a vampire
are directly tied to feeding on other characters; in short, for everyday you go without
feeding, vampirism takes a greater hold on you an turns you into even more of a
monster.This means that the benefits you receive from being a vampire become stronger, but
so do the drawbacks. If you reach the end of the cycle without feeding you’re
prevented from interacting with most other characters in the world.
    Feeding is simple: Find a sleeping character, and activate him or her. You can either
wake the person and converse or feed. (If you are sneaking, you can choose to pickpocket
your target.) Choose to feed, and you’ll chow down. Don't worry; this doesn't kill
your victims, and it wont turn them into vampires. Each person is a never-ending buffet,
so long as you don't wake your target up.
    Remember that others won't react well to seeing you feed. It will be treated as a
crime, so pick your meals carefully. Since most people sleep behind closed and locked
doors, beggars are a good choice. Find them sleeping on mats outside in town, and make
sure there are no guards around before going to work. Joining a guild, such as The
Fighters Guild, Mages Guild, The Dark Brotherhood, and/or The Thieves Guild gives you
access to dozens of sleeping characters. Again, just be sure no one is watching.
  Feeding has a direct effect on how much power you have as a vampire.


The good side of vampirism:

    At its basic level, vampirism gives you bonuses to several attributes (+5 to strength,
willpower, and speed) and skills (+5 to acrobatics, athletics, destruction, hand to hand,
illusion, mysticism, and sneak). You're also granted a resistance to disease and
paralysis, and a small resistance to normal weapons.All of these factors combine to make
your character a bit tougher then normal.
    For every day you go without feeding, your boosted skills and attributes increase by
an additional 5. Your resistance to normal weapons increases as well.
    For every day you spend as a vampire without feeding, you are granted a new power.
While they may not directly help in combat, each power has a specific use and can be very
beneficial.


The bad side of vampirism:

These bonuses come with a price. At the start, you'll only have a weakness to 
fire. Each day without feeding increases this weakness, and other drawbacks are 
introduced.
  If you've fed within the last 24 hours, you’re safe from the sun. Any longer
then that, however, and you'll start burning whenever you are exposed to sunlight. Each
day passes without a feeding increases the amount of damage the sun does; cloudy or rainy
weather will help some, but you're in danger any time you set foot outside.
  The worst part of not feeding comes after a full four days without blood. You may notice
that for every day you don't feed, your appearance changes. Your face becomes gaunt and
pale, your teeth start to look like fangs, and your eyes turn a bright red. Well, you're
not the only one who will notice these changes.If it's been long enough since your last
feeding, everyone else will recognize you for what you are. Most characters in the game
will simply refuse to interact with you, turning you away every time you try to talk to
them. This can make playing the game difficult, as it prevents you from buying and
selling, and sometimes prevents you from getting new quests.


Hope this helped!