Mayonnaise and plain yogurt share many characteristics, from their paleness to their thick, creamy textures. However, they're different substances in their makeup. Yogurt is simply milk that's been acidified and thickened by beneficial bacteria, while mayonnaise is an emulsion of eggs and oil, with lemon juice or vinegar added to freshen its flavor and counter its richness. You can't turn yogurt into mayonnaise no matter how you try, but it can sometimes be a complement or substitute.
The Virtues of Yogurt
Given that yogurt is made entirely of milk, and that mayonnaise is made almost entirely of oil, it's unsurprising that a tablespoon of mayo contains more fat than a full cup of yogurt. It's also packed with protein, vitamins and minerals, most of which are completely absent in mayonnaise. When you're preparing your favorite sandwich or salad, you can easily reduce its calorie count and up its nutritional value by mixing some plain yogurt into the mayonnaise rather than using all mayo.
Yogurt contains thin, watery whey, which tends to leak out over time. If you're going to use it in salads or sandwiches, they should be served immediately rather than made up ahead. In this case, "less-natural" mainstream yogurt is a better choice than your premium brand, because it's thickened and stabilized with gelatin. You can also drain your yogurt overnight in a colander lined with a pudding bag, which presses out most of the whey and leaves a thicker, creamier yogurt.