EDIT : Fixed, the ux0:app folder you mention on the install instruction is incomplete, it mention folder "APTR1007" but it is actually "APTR10007".
Maybe if i installed the vpk first it would have been already created and i would have seen that it was the right folder
I also have the exact same screen. Everything was installed as described, extracted the Media.zip from Steam files, copied to ux0:app/APTR1007/Media
Then copied portal_pak_dir.vpk to ux0:data/AptrRecon/ (i have the french version of the game, i guess i have to do it also ?)
then i installed the vpk using Vitashell. I launched the game, and i have this screen as dexter2939