Donna asked via Twitter:
"i have a ? for you. Why vinegar in Red velvet cake? And why mix it with bkng soda before adding to batter at the end?"
Red Velvet Cake is an interesting beast. It's kind of a traditional cake, but, as with many traditions, a lot of people aren't really sure where the traditions came from. I'm not an expert on the history of this particular cake, but I've pieced some things together and I think I've figured out what happened.
Red Velvet Cake is a rich, lightly chocolated cake that is bright red. Nowadays, we use a few gallons of red food coloring to dye the cake that color, but it's not always been that way. Quite a few people seem convinced that an older form of the recipe called for beets instead of food coloring. Kind of like a carrot cake, but with beets and chocolate.
The thing about beets is that the common variety that I see has a purple-ish/red-ish tint to them due to the pigment betain. Like anthocyanins, the pigment in blueberries, betain changes to more of a blue-ish color in an alkaline environment. This means that if you mix in a baking powder or baking soda into a beet-filled cake, it will be less Red Velvet than Blue Velvet, which is great for shoes but not as good for cake.
To work around this, people would mix the baking soda and vinegar and, while it's reacting, fold the bubbles into the cake mix, much the same way you would fold in an egg foam to a batter to lighten it up. I can see the theory here, as the baking soda would no longer be alkaline but you have a bunch of bubbles, but I just don't see this as being a good way to provide lift. The weight of the batter would almost certainly crush whatever bubble structure would be formed by the baking soda and vinegar, because there's not a lot of structure intrinsic to that reaction.
Egg foams have relatively tough, protein enriched structure to help it stand up to a cake batter, and even egg foams are delicate critters. I think that, if you want to go that route, go ahead and make an egg foam to fold into the cake, rather than mucking around with the vinegar/baking soda combination.
Now, if you making a Red Velvet Cake, and you decide to go with the red food coloring, then you shouldn't need to go through hoops with the leavener. Modern dyes for cooking are reasonably resistant to little changes in pH.