You could use a small square table. What area is this in your home?

I am sorry, but in my opinion the mirror is sort of big for that spot and anything you put under it should have open legs because of the vent. If you want to leave the mirror, how about a wrought iron stand with a plant in it? Plant should be bushy like a fern or a pothos.

What room is this in and can we see pictures of the rest of the room? What is your reason for putting the mirror there? How are you wanting to use the mirror/space?

Agree with others, the mirror is too large for that area. A small, open corner table would be nice in that spot with matching prints on either wall.

That's a great corner for a plant.

Remove the mirror and leave it alone.

That mirror is too big and it unbalances the corner. Do something like this instead: