Pour vegetable stock into a saucepan over low heat. Cover and keep warm.
Combine hot water and saffron threads in a small bowl.
Heat 2 tablespoons oil in a stovetop pressure cooker. Cook and stir onion until softened, 3 to 5 minutes. Add Arborio rice; cook and stir until toasted and coated with oil, 2 to 3 minutes. Increase heat and pour in wine; simmer until alcohol has evaporated and rice has absorbed wine, about 2 minutes.
Stir vegetable stock and saffron mixture into the pressure cooker. Seal according to manufacturer's instructions. Increase heat to high; cook until cooker whistles, about 5 minutes. Reduce heat to low and cook for 4 minutes.
Remove cooker from heat and release pressure according to manufacturer's instructions. Remove the lid. Stir in 1/2 cup plus 2 tablespoons Parmesan cheese, pancetta, and butter. Let stand until rice absorbs remaining stock and Parmesan cheese melts, 2 to 3 minutes.