I made a batch of peach salsa last night but forgot the vinegar. Can I add the vinegar and recan the peach salsa. It has the sugar, just forgot the vinegar
The key here is really how long it sits out at room temperature before you realize. The vinegar is acidic, so presumably it's part of bringing the pH to a safe one for boiling water canning. Without it, the salsa won't be safe to can like this.
If you realize right away and recan immediately, it's basically just cooking it extra. It may not be as good with the extra cooking, but it'll be safe. Similarly, if you immediately refrigerate then reheat, it spends a bit longer in the danger zone, but it should be easily within a safe margin.
On the other hand, if it sits out for hours, all bets are off. You made something not safe to store at room temperature, then stored it at room temperature, so you really can't know it's still safe.
Maybe is a good way to start any answer to this question. As far as food safety, there is no way to know if any pathogens have propagated in the jars without testing. As far as recanning, you may find your salsa will lose some of its color and flavor after cooking a second time. The pectin from the peaches that is thickening the jam may become weakened when cooked again, so if you are considering recanning your salsa, you may want to add a small amount of commercial pectin to help retain the consistency from your original batch.