I think the problem is that what you want (a vanity bitcoin address) and the method you want to use (SHA256(string+n)) don't make sense together and that is what people are trying to tell you. Mainly it doesn't make any sense to start from a starting seed to generate the vanity address, if you want such an address then simply start from a random key.

I think you are talking nonsense how do you explain key generation? Do you think the random key is generated out of nowhere? Random key generation also needs input without input there is no output also to say that random key generation is the only secure way to generate is a real security risk since all random numbers cant be trusted havent you heared of the weak random seed generated by android clients?

You said something about forgetting and being able to recover it from that seed which again doesn't make sense if you are generating a vanity address because depending on how many fixed starting characters the address has the process can take a very long time and recovering it this way is not practical. Not to mention that using a starting string that you can remember suggests weakness in the randomness.

It makes perfect see the above reply.

Here is a suggestion, why not generate a random entropy then start your vanity address search from there until you find the key that corresponds to your desired address. Then take the private key to that address (256-bit) and encode that using an algorithm like BIP39 to a set of words that you can easily store or remember (although it is not recommended to only rely on your memory for backup).