Currently you are forced to use Program Files/VSTPlugins, a folder where none of my current VSTs are installed.
Having the option to change where OBS looks for plugins would be super fantastic...Unless it is buried in an .ini that I haven't looked at.
The topic is a bit older, but still there is no option to set a path for VST plugins in OBS. Since you come across this thread through the Google search and there seem to be barely any workarounds, my current, though not the most beautiful solution for it:
Under Windows choose one of the paths in which OBS searches for plugins by default, for example C:\Program Files\VSTPlugins. Create a symbolic link in this folder which points to the actual VST folder. This can be created in the Windows CMD console with "mklink /D <NameForYourLink> <TargetFolder>". For example in my case: "mklink /D OBSPlugins D:\VSTs\". OBS then follows the link and finds the plugins.
As long as the search path cannot be changed, this is a simple solution. Only checks if other VST programs are not confused by the link.