I mix and develop glaze recipes for midfire stoneware (fired to 1220°C). The colorants in pottery are mostly iron, copper, cobalt, tin, titanium, chrome, and a few other transition metals. Many of these materials are somewhat toxic in the raw form but safe in the finished work. These colors usually make up less than 3% of a glaze recipe.
Vanadium is a colorant that potters seem to fear without good reason. I find resources such as MSDS sheets to be very unhelpful on this subject.
The heart of my question is: in your humble-I-am-not-going-to-sue-you opinion, is vanadium (specifically tin-vanadium) safe enough? How about relative to cobalt?