Hi I have recently really been getting into cooking however the girl I am dating has allergic reactions to vinegar, which is in a lot of sauces and stuff.  Looking for possible substitutes that might work when recipes call for this, currently use lemon juice but sometimes that tastes weird (For example I tried to make ketchup since she can't have kraft ketchup which she use to love, however lemon juice tasted quite horrible for my try at it).  Let me know what I should be using! thanks:) 
Brew your own wine stores sell "acid blend" crystals which include all three of the main organic acids: tartaric, malic, and citric. The ratios differ between brands. You could try mixing up a bit of acid blend in water to make a solution that is 5% acid, similar to the acid level in vinegar and see how you like it. Some wine shops also sell the acids separately which would allow you to adjust the ratios to your own liking. Most fruits are a combo of malic and citric acid, so if it were me, I would make a 65:35 blend of malic and citric and leave out the tartaric, as tartaric acid is pretty rare in nature. Malic acid is a good flavour blender, and its sourness lasts longer. Citric acid is sharper, fresher, and quicker. In situations where you want a lot of citric acid character one of the citrus fruits will give you more complex flavor. Limes have the strongest flavour, lemons are a bit less strong, but oranges are fairly mild and tend not to dominate. Orange flavoured bitters are handy in cooking. A solution made with a 50:50 blend of tartaric and malic acid could stand in as a replacement for wine vinegar. One made with just malic acid could stand in for apple cider vinegar. 