The sauce that I will describe here, I have used for years on grilled meat and I have never had a complaint about the taste of the meat. I start with a good quality Italian Dressing. I prefer a brand that is packed with veggies and spices (pic # 1 above). Notice that the transition from oil to vinegar is clean and appealing. Pic # 2 shows a dressing that is devoid of ingredients and, I have no Idea what that is floating between the oil and the vinegar. Needless to say, I won't be using this dressing for anything. The process I use is a fairly simple proceedure however, the bottle must be the squeeze type. I stand the bottle on its cap for a few minutes, remove the cap over a cup and squeeze out a cup of dressing being careful not to agitate the contents.
When I accomplish this successfully, I can remove everything but the oil which, in this case, happens to be soybean oil. Any remaining vinegar/spice will be consolidated into another bottle of salad dressing or stored for use in another recipe or sauce. I strive to waste as little as possible.
I pour 1 cup of the vinegar/spice mix into the sauce pan. To this I add 3 tsps of sugar, 1 tsp of minced garlic and 3 tsps of finely chopped onion. If the sauce requires salt, I use chicken bullion instead to bring the seasoning up to taste. The final step to making the sauce is to apply low heat until the sugar melts, too much and the sauce will burn. Once it starts to bubble, I monitor it closely to reduce it slightly so that it thickens. Once that is accomplished, I turn off the heat and cover the pot. The sauce is ready for grilling.