Solved. I replaced the file "steam_api64.dll" of 238KB for another file of the same name, but different size: 1040KB (found in others VR games). It worked.
I am in the process of downloading this now. The only steam_api64.dll I have that is 1040KB is from the cracked DOOM VFR game (also from here). Will let you know if it works... Edit: Yep, it works!
Edit: Yep, it works!