You can’t. HL2, no matter where you install it from, uses the original texture. Ep2 uses phong shading, which HL2 doesn’t have, to make it look all pretty.
You have to get your hands on the Ep2 engine conversion to get it to work. And simply copy-pasting the texture won’t work, either, because the ep2 Vortigaunt uses a brand new model.
I thought that when you download HL2 via STEAM that it comes with the Ep2 vortigaunt? So couldn’t I just replace the texture and model?
No on both counts. Steam HL2 still runs on the HL2 version of source.
Ah well, I’ll just wait until the aforementioned mod gets a patch out so I can replay HL2 with HDR 