Is there anyway to have the installer install to a temp file? I'd like to then take that temp file and install is as a mod in Vortex. Thanks.

No need, once you have had GOG Galaxy install the game you can make an archive from the files it has installed into ../FalloutLondon, Ignore the _* folders, they are the config files and need placing manually, also ignore everything in the root apart from:
https://i.imgur.com/w5qkln3.png

zip that up and install as 'engine injector' in Vortex

diziet

edit: forgot to explain, the initial GOG Galaxy install of FOLON doesn't actually transfer the files to the F4 folder, it installs the files to a separate Fallout London folder. It is the 'installer.exe' file in this folder that is (I believe, since I didn't actually do it!) launched by the FOLON 'Play' option. I imagine that after the first 'Play' the installer.exe alters the launcher .

Interesting. That's how I initially installed FOLON. But I deleted the FalloutLondon folder because it was so large.