I might have answered my own question: Vit C degrades at 190 C or 374F

okay--
so my bottle was on the vent for 30-40 minutes during which time I turned on the oven (so part of that is warm up time) and then heated my food. Only the bottom part of the bottle was hot (and it's plastic so not a good conveyer of heat), and the oven was only set to 325F.

though, I do worry that the concentrated heat output of the vent might have been more than 325? I don't know. but, this is probably the best answer I'm going to get.

temps taken from wikipedia:
"Vitamin C chemically decomposes under certain conditions, many of which may occur during the cooking of food. Normally, boiling water at 100°C is not hot enough to cause any significant destruction of the nutrient, which only decomposes at 190°C, despite popular opinion. However, pressure cooking, roasting, frying and grilling food is more likely to reach the decomposition temperature of vitamin C. Longer cooking times also add to this effect, as will copper food vessels, which catalyse the decomposition.[32]"

http://en.wikipedia.org/wiki/Vitamin_C