I've just opened one can of canned corn. Besides corn there is always some liquid inside — just like in case with any other canned vegetable. So what is that liquid? How would you call it? Juice, bouillon, soup?
Depends upon what it actually is.
Salt water: brine.
Sugar water: syrup.
Thicker stock: gravy.
In a can of pineapple I would call it juice. But when it comes to corn and such vegetables I think I would possibly just call it liquid. (I'm sure you can still call it juice. But to me juice means something that comes from the fruit/vegetable when you squeeze it).
I would call it excess fluid, because that implies it's nothing you'd want to add to a recipe (in fact in recipes, it's written to drain the excess fluid). However you can't tell your friend to drink the excess fluid, so I'd say corn juice, for lack of something better.
A general word for this is "liquor" (OED meaning 4. "The water in which meat has been boiled; broth, sauce; the fat in which bacon, fish, or the like has been fried; the liquid contained in oysters.", though I admit that that definition doesn't cover vegetables).
However "liquor" has a more common meaning, which may be thought undesirable in this context.