>medieval

When, because that's about a thousand years.

>to wear a leather vest+skirt combo over a full chainmail suit?

I don't think I've ever heard of such a thing.

>Or would it be more common to just wear some clothes over the mail if they wanted to show the colors of their master?

You might be thinking of a surcoat, jupon, or tabard, depending upon the era. These could be simple colours or embroidered, and while a squire would want to show his allegiance, for most of that era a man-at-arms wouldn't get the same sort of consideration, and many of them would be lucky to get a 'uniform'.

The two governing factors here are likely padding and heat. If they're only wearing one leather article of clothing, it would be under the mail to provide extra padding to both further spread out the force of a blow and to provide better protection in the event a blow got through the chain links.

If they're wearing a padded aketon/gambeson and then mail, and then a leather waistcoat, that's basically wearing two winter coats with 20 lbs/10kg of weight in the middle. The heat load is going to be severe.

In battle, if they're wearing their knight's colors or badge, you'd be more likely to see it on a shield, a strip of cloth tied around an upper arm, a light surcoat/tabard, or a hat/helmet badge. But, they're not going to be in full harness most of the time, and a leather coat with a badge could be reasonable "around town" uniform/light armor, just like they might carry a large-but-not-huge knife instead of their battleaxe when going into town. As.a garment, it's an extravagant use of leather, but it's not crazy.