Is this vanity light dimmable?

Yes. It is dimmable, but only with dimmable bulbs and a compatible dimmer switch. Standard bulbs will flicker or hum on a dimmer.

What bulbs does it take?

It has three sockets for standard medium-base bulbs. Check the listing for the maximum wattage per socket before buying, and plan on buying bulbs separately.

Can it be mounted upside down?

Most three-light bar fixtures like this can be flipped so the lights face up or down. Confirm the mounting bracket allows it before you drill.

Will it fit over a 24-inch mirror?

It is 24 inches wide, so it matches a 24-inch mirror closely. Over a wider double vanity it will leave unlit gaps at the ends.

Does it need a junction box?

Yes. It is direct-wired, so it needs a wall junction box and a live circuit. If there is no box behind your current fixture, add one before installing.