I wanted to ask if a small aquarium pump and airstone is enough to oxygenate a couple hundred gallon vat or tank full of fish?

When it comes to aeration:

Nothing comes close to "flooming" - This requires nothing more than a water-pump which can lay flat on the pond or tank bottom and force water upward to the surface of the vat.

It is not important, or even desirable for the water being "floomed" to break the surface - we DO NOT want a geyser.
All we want is a gentle swell at the surface where the water looks like it's "bulging" over the pump location. You see, oxygen exchange occurs only in contact with atmospheric "air" which is 21% oxygen. Venturis and other bubblers simply create agitation of the water and the bubbles lift the water to the surface for gas exchange.
Even better, a water pump forces large amounts of water to the surface for gas exchange, and it does so "simply" and very quietly. Flooming is ideal in retailers tanks because, unlike air bubbles, flooming causes less surface distortion so the consumer can still see and buy fish.

A water pump lifts more water than an airstone could EVER lift to the surface for gas exchange.

Almost any brand of pump can work. You will find some pumps "stand up" better than others. It can be disastrous if the pump lays over and stops "flooming" the surface.

Also, if the pump DOES "geyser" the surface the risk is increased that the pond or vat could be pumped dry by the pump if it directed water over the edge and onto the ground.

So, a gentle flooming in any tank or vat is superior aeration. And it's as simple as dropping in a submersible pump and aiming it at the heavens.

Erik -

This information has gone into wide spread use especially at the retail level because the fish for sale show up better without a cloud of bubbles on the surface. However, despite being a common thing, the pundits will not even try it because it must be patently wrong. Why try something they "know" is wrong? Remember, whatever they know, is the one and only "right" way.