I don't understand the goal here. If you want to create many vanity addresses from the same public key, then all of them will have one private key. So this isn't really for many users, it is if one user wants many vanity addresses and that they are transparently connected to each other for anyone to see.
You can't make multiple addresses from one public key, that's not how addresses work. It is only one public-private key pair for each address. What he is doing is that he would be doing split key generation where the public key generated is added to the public key owned by a trusted third party so that a final public key can be generated and its address generated.
I know, it is what I meant to say. I just assumed that like in RSA keys, if you start with one public key and change just the exponent of it in order to change the hash, then it's private keys of all of these new keys would have the same factors and be able to calculate the private exponent for all these public keys.
Assuming that during key generation factors are not discarded and just private exponent kept.
I made a lot of assumptions here based on my understanding of RSA keys and how you could generate different hashes for public keys with same module by just changing the public exponent, so that the owner of private keys could generate a private exponent corresponding to the new public exponent. I assumed it is a similar process and that all of these public keys would be (visibly) connected in such a way. Am I correct?