Hello
The biggest problem with air vents is that they blow dry air. Unless you are growing a cactus or a select other few plants your plants will like moisture in the air. Humidity requirements are different for every species however indoor plants are traditionally tropical, which means they are naturally from a very humid environment. Keep the humidity up with a humidity tray. They are cheap and you can buy them just about anywhere that sells plant supplies(use pebbles or something to keep the bottom of the pot out of the water you dont want root rot). you can also just place a small water dish next to the plant. Refill as nessesary. An air conditioner effects tropical plants negatively because they are warm weather plants. So dont blow cold air at them in the summer put them outside where they can get the full benefit of sun and warmth. Make a nice little stand for you to enjoy them on while they are outside and watch them grow.

Bengro,

If you are relying on sunlight you probably don't have a whole lot of choice as far as the placement of your trees. If you are using supplemental lighting then make sure that your trees are not in the direct flow of air from the vent.

To add to and clarify some of hooks statements, air conditioning is not just an issue of temperature it also has a drying effect. And pebbles in a humidity tray do more than simply provide support, they vastly increase the surface area from which water can evaporate, thus increasing the efficiency or the tray.

Thank you very much.

You can also enclose the area on 3 sides of your tree, using a folded screen made from cardboard or the like. This creates a sort of "microclimate"; it protects the tree from drafts and helps hold humid air around the tree. See sketchy drawing at the URL below.

Here is a link that might be useful: Indoor Bonsai Survival