Vanity private key? That is really stupid, because you can just take a random private key, like 5XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX (however long a private key is), and just change the first few digits to what you want. My vanity private key (not sure why you would want one) would end up as 5DarkStarXXXXXXXXXXXXX.
If you mean vanity address, up to what length are you willing to generate for free for the first 3 people?
No nobody can change first 2 digits according to their wish. It wont work that way.
All you have to do is to brute force the preferred prefix and wait for it to get found by the owner.
In this case OP is offering (maybe) a split-key vanity generation service where you give a public key and he generates a generic private key of it and you derive the exact private key for the vanity. This way nobody except you will actually know the private key.