I will sometimes add stain to a tab when seeing a patient at the lab. It's also a great way to learn MIYO.Probably better off treating a shade tab with MIYO and freezing that shade adjustment with vlc. Would be easier to copy that cuztomized tab back in the lab. Just try to get the value right as best as you can chairside, which may shift when you light cure. Burn off of vlc material would wreak havoc on your well placed effects. Tanaka used to have a kit for this purpose, worked very well for modifying a tab but was quite touchy to burn off and fire. Did use it numerous times successfully , not sure if its still even available. Jmho and good luck!
I may go that route instead.
I can't get a decent pic from the Drs but the Trios is pretty good.