I recommend trying olive oil in your chocolate brownies at some point in the future. It turns out to be a delicious substitution, particularly if you add a little coarse sea salt on top of the brownies just as they come out of the oven. I have done this with the Ghirardelli brownie mix (top rated) several times and it's great!

Vegetable oil is simply a broad classification for oils (sight be soy, canola, rapeseed, sunflower, corn, etc)derived from a vegetable source, as opposed to an animal source (lard, butter, ghee, etc.)

Sometimes canola can have an odor/flavor profile that's a little off-putting - but in a brownie mix it will be just fine.

Yes. The only difference between those oils are the sources from which the oil is derived. They're equal to each other in baking. (Canola is from rapeseed, while vegetable oil is usually mostly from soybeans.)

Yes. They are, for this purpose, interchangeable.