Place the cold rice in a bowl. Drain or pat your cold stir-fry to get as much of the sauce off as possible. (We like to use Orange Chicken.) Chop up the stir-fry into small pieces. There can be a mixture of rice and veggies. Add it to the bowl of rice. Crack an egg over the top and stir everything together.
Now it's time to get messy. Get your hands in the bowl and roll little balls of your mixture. When you have a small ball, roll to completely coat in the panko.
Heat up the oil in a pan and fry the balls until crispy.
In another small saucepan, mix the soy sauce and vinegar. Simmer on low for 15 minutes. Serve the arancini with the dipping sauce.