Heat oil in lg skillet over med-high heat. Ad onions and salt. Cook, stirring often, until they begin to brown. Add broth, scraping up the bits. Simmer until liquid is almost gone. about 20 min. reduce heat to med-low, cook another 5 mins. Stir in onion powder, then vinegar. Cook until liquid is evaporated. Remove from heat. Let cool 20 min. In bowl, combine sour cream and yogurt. Stir in onions, mix well. Refrigerate 30 min.