I think it largely depends on what kind of feeling you want to evoke when the character is having the vision, and what kind of vision it's supposed to be. The more chaotic a vision is supposed to be, the less clear the visuals should be (lots of colour, movement, images merging into one another). On the flipside, the clearer the vision is, the clearer your depiction needs to be - close-ups of eyes, clear drawings, clear text, etc. A good way of doing it is a colour change from the rest of the comic.