If you talk about project https://github.com/JeanLucPons/VanitySearch/, then take a look at file
https://github.com/JeanLucPons/VanitySearch/blob/master/Vanity.cpp
There in line 306 the process of setting initial key starts. If you ignore the next like (sha256), and set your desired key, it should work. Ah, and of course do not use 'rekey' option.