First you have to obtain a application form. Often with secret and exclusive cults this is only available from an existing member. If so then you have to somehow attract or capture the attention of (a) member vampire(s) and convince him (them) to let you apply. How to do this is the subject of a separate question and only for those who are really serious.
Fill-out the form according to the rules (there maybe many of these), submit the application fee and mail it all in. NOTE: the act of applying doesn't assure induction or even a response. The fee is non-refundable and (quite honestly) there are many scams out there that are simply into ripping you off for the application fee. Beware of any society or cult that allows multiple or repeated submissions.
If you receive a response. You will be expected to memorize it and them destroy it. Follow whatever instructions it gives. What will follow will be a series 'look-sees' or 'tests of your intentions'. A look-see is merely the member-ship or some sub-set there-of having a look at you, trying to make-up their minds whether to include you or not. Perhaps the first few of these you will be oblivious to. In this case your instructions will be to simply 'await further instructions'. Then you will be sent to sites of recent cult activity and observed for your reaction to what you find there. If you react adversely this may be the end of your application. These 'visits' may then advance to the level of tests, where you are expected to perform some compliant activity. Usually this means 'cleaning-up after' them. They will want to 'distance themselves' from their own handiwork and as you are 'as unconnected as possible' you are the best one to do the 'mopping-up'.
At some point these test (may) advance to the point where they become an act of 'unrecoverable complicity', what those in the criminal under-world refer to as 'making your bones.' You will be expected to do something that makes it impossible for you to renounce or betray the society. BEWARE of any society or cult that demands this level of complicity. Best you seek-out some society that doesn't require such a commitment.
Then you will be allowed/required to undergo some induction ceremony, rite-of-passage, baptism, etc etc etc. There may be a long series of these as you are allowed higher up the levels of the cult.
As vampires are mythical creature will little to no evidence proving their physical existence, I doubt there are any real vampire cults in the world. I would imagine however that there are sects or...