This 2011 study examined the antioxidant capacity of fractions of vegetables and pointed to an interesting observation, namely, that in vegetables with a number of flavonoids, the ascorbic acid had over ten times the bioavailability of synthetic vitamin C. Careful analysis revealed that the tannins and flavonoids such as quercetin were also considerably enhanced by vitamin C.
References:
Ji L, Wu J, Gao W, Wei J, Yang J, Guo C. Antioxidant capacity of different fractions of vegetables and correlation with the contents of ascorbic acid, phenolics, and flavonoids. J Food Sci. 2011 Nov-Dec;76(9):C1257-61.