As Wikipedia states (with good sources):
A Vidalia onion (/vɪˈdeɪliə/ or /vaɪˈdeɪliə/) is a sweet onion of certain varieties, grown in a production area defined by law of the U.S. state of Georgia and by the United States Code of Federal Regulations (CFR).
The varieties include the hybrid yellow granex, varieties of granex parentage, and other similar varieties
As far as I understand, there are varieties with puprlish tint in the mix. The most defining thing about onion being accepted as Vidalia is taste, capability of growing well and not developing sharp taste when grown on low sulfur soil around Vidalia, Georgia. If taste is sweet and mellow as you would expect it to, then color is probably OK and onion is within what state of Georgia allows to call an vidalia onion.
It is also possible that what you got was unplanned crossbreed with red onions from near field. This can happen if onion is grown from seeds, especially seeds gathered by the farmer himself, as seed selling companies tend to be very careful about that. In that case, these onions should not be sold as Vidalia onion - but if they are half-Vidalia-accepted-variety and grown in proper soil, taste should be pretty close to what you paid for.