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.

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.

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).