You're not going to experience Vitamin D toxicity from being exposed to the sun, even if you're outside all day on a clear day in summer. A chinchilla that lies in a sunny patch for 20 minutes a day? There's absolutely no chance that the vitamin D is going to have negative effects (now, there are obviously other risks associated with leaving a chin in the sun, such as the possibility of heat stroke). Pretty much the only way you can overload on Vitamin D to the point where it has negative effects is if you take mega-doses of Vitamin D supplements.
I imagine that the exposure to a little extra Vit. D through the sunshine is probably beneficial, if anything. However, this doesn't mean you should let your chin's cage sit in the sun, as it would be easy for the chin to overheat. I would strongly recommend NOT putting your chin's cage in a sunny spot, even for short periods of time.