Can I use something other than lemon juice in this recipe? Is it added for flavor or is the acid necessary for a chemical reaction? If the latter,...

Acid in candy making can serve two purposes. If the acid is added to the sucrose and then heated, it can act as an interfering agent (to prevent crystallization). It does this by partially inverting the sucrose (breaking it into its two component parts, glucose and fructose which do not crystallize as readily as sucrose).

Added after the syrup has been cooked to the desired consistency, acids are used for flavor, tartness and to balance sweetness. Since different acids have different properties, flavor of course, but also how the tartness registers on the tongue, it is inadvisable to make substitutions unless you know beforehand exactly what to expect.

IMHO, I wouldn't sub vinegar for lemon juice here.

Typically lemon juice is added to caramel to invert (ie split) some of the sucrose molecules into glucose & fructose molecules. This helps prevent the sucrose from crystallizing. This particularly recipe uses corn syrup which is an invert syrup so it shouldn't *need* the lemon. I vote leave it out.

From what I've read the lemon juice does play a chemical process role. Not sure if a vinegar would do the same. However, I've made many caramel without lemon juice and they turned out great. So I'd sooner eliminate the lemon juice than replace it