first try my suggestion, secondarily, I only have two vita pstv's that I hacked, one, with the initial version of mlthaku, and then the other with the new version of mlthaku, then, I updated the other one with the old installation of mlthaku with the new one.
you can try the "sd2vita" plugin and add it manually to the tai/config.txt folder and add the line to the config.txt, since mlthaku prefers internal memory. henkaku.xyz/go/ will install to official memory card the "molecule", needed to install henkaku enso.vpk, then you will add the "sd2vita" plugin to the tai folder and the path to the config.txt in the tai folder..................when you restart your vita, then your official memory card will be on ux0 as normal, but your Micro SD will be on "uma0", then you install "USBMC" and after you restart your Micro SD will be on ux0, then you should be finished, that is only a theory, I don't have a fat ps vita to test this out, but in theory it should work.
(the answer is yes, with no internal memory, you need to use an official memory card, and you can't use mlthaku since it uses internal memory, your forced into using the "sd2vita plugin")
(next best thing to do, is to assume mlthaku has programming to detect a vita without internal memory, and to follow the installation guide, with an official memory card, and maybe it will be used like internal memory) (you will just have to test this out)