Try specifying the path to the .dll or change the directory to where the .dll is ( C:\Program Files (x86)\Steam\steamapps\common\VRChat)
The /u switch should unregister the file so you may want to register it again.
regsvr32.exe /u C:\Program Files (x86)\Steam\steamapps\common\VRChat\UnityPlayer.dll
to register:
regsvr32.exe C:\Program Files (x86)\Steam\steamapps\common\VRChat\UnityPlayer.dll