Bring cream just to a boil in a small saucepan over high heat. Reduce the heat to low and keep the cream warm.
In a large high-sided saucepan over medium-high heat, dissolve the sugar with 2 tablespoons of water. As the sugar mixture begins to bubble, watch for crystals developing on the inside of the pan just above the liquid. Using a pastry brush dipped in water, brush the inside of the pan right above the crystals so the water drips down and dissolves the crystals back into the liquid. When the sugar begins to brown, occasionally move the pan to swirl the liquid gently and cook it evenly. Continue to cook until the mixture is dark golden brown. The total cooking time will be 8 to 9 minutes. Remove the pan from the heat.
Very carefully add the hot cream to the sugar mixture a few tablespoons at a time. The liquid will bubble up dramatically. Stir the sauce and cook for 1 minute. Add the vinegar, lemon juice and 1/2 teaspoon salt, mix well. Pour into a heatproof bowl. You should have about 1 1/4 cups of sauce. The sauce can be made several weeks ahead, covered with plastic, and stored either at room temperature or in the refrigerator. If refrigerated, warm in a microwave oven before using.
Serve on top of vanilla ice cream.
