I would like to give drivers the option of adding fuel stops in a Vintage F1 series, and I assumed the mod would be setup for it. But, I don't see anywhere in the Garage screen that you can. Is it up to the author of the mod to allow it, or is it a server setting ?
It's up to the mod. Being a vintage F1 mod it's probably been disabled. The HDV line is this one: [PITMENU] Fuel=1 // Whether fuel pit menu item is available (recommended); default=1 With some other lines determining fuel levels available, while lower down in the PITMENU section you could change how quickly the refueling happens with the FuelTime parameter. Of course if you were to make such changes you'd need to repackage it into a new mod and redistribute it to your drivers.