Hi Max,
You can load VSTs (not VSTi) into game code by using System::loadPlugin and then System::createDSPByPlugin. Please note that you will need the 32 bit and 64 bit versions if you are building different versions of your game.
https://www.fmod.com/resources/documentation-api?page=content/generated/FMOD_System_LoadPlugin.html
If you wish to be able to use a VST within FMOD Studio application it will need to be built it in a specific way to work with FMOD Studio and to expose its properties.
https://www.fmod.com/resources/documentation-studio?page=plugin-reference.html
Thanks,
Richard