Hi & welcome to Gardenweb! Sorry your plant is ailing. From what you said, these are the important clues.

"my window sill is right above the heat vents."
This sounds like it's getting directly blasted with hot, dry air occasionally. Plants don't like that, intermittently, in a generally cold room.

"The soil was slightly moist at the base after weeks of not watering."
This sounds like the roots have rotted and are unable to deliver moisture to the plant. So the foliage wilts. Adding more water won't help that, just exacerbate it. This is what overwatering means but it's not your fault. It's the soil holding too much moisture, so the problem is really underdrying.

If there are no tiny particles in a pot, it will dry soooo much faster. People who lose plants because they forget to water would have a hard time with this, but if you're the more common 'overwatering' type, which it sounds like you are from reading about your Aralia, this can mean the difference between plants that stay alive vs. those that die. I used to kill plants by 'overwatering' too until learning how to fix things through the great info here at GW.

I might recommend an alternative source of water other than tap water, such as rain, melted snow, condensate from dehumidifier or A/C, distilled. It makes a huge difference if tap water is making one visibly ill (which it usually does over time.)

Due to genetic reasons, these Aralias have been renamed Schefflera. Your plant's correct name (for today anyway,) is probably Schefflera elegantissima 'Cultivar name here.' That gives us more clues about its' preferences. Mainly, that the roots can't survive sitting in soggy, dense, airless soil (if we didn't already know that when it was Aralia.) To fix that, we need to give them something much more chunky, porous, airy. Most plants can survive getting too dry once in a while. OTOH, getting soppy wet too often and taking forever to dry out is fatal to most.

More light sounds like a good idea. I have several cultivars of S. elegantissima that grow outside, with the sun hitting them for a few hours in the afternoon. Another pot that is an east windowsill, so direct sun for a few hours in the morning.

I would encourage you to paste what you've written into a post on the house plant forum, to gather all of the excellent advice possible there. This informative post there will make it an easy trip via a click. (At the top of the post will be a link to the house plant forum page.) If you can add a pic, you'll get even more specific advice. Sending good vibes!

*checking after typing isn't the best, I think I spoke incorrectly above about it being renamed to Schefflera, sorry. Your plant may be Polyscias. A pic would also help to ID it.

Just wondering if there's as way to encourage foliage when it has only sprung out in awkward spots-will they continue to pop from the trunk?-a couple were obviously dead but a couple had green cambium under the bark. I know you can lightly nick through the bark with a sterilized blade with some trees and it will stimulate limb growth- anybody tried this or am I just impatient - they will be a fab forest if I can work this out- one thing I forgot to mention that these are all trunk propagated, but the stump left is at least3-5 inches- isn't that a little long

I've never seen an aralia die slow. I've seen complete leaf drop, that's called "shattering" I learned today just this summeri lost a trunk of an extremely vertical upright, bot once I cleaned it up I liked it better- I like the law of three- no even numbers, but, when I gently eased it from its pot,it was full, I mean FULL of carpenter ants- almost like an inch thick layer of pea gravel.that pot got tossed- I usually use plastic mesh for needlepoint to cover the drainage hole but I missed this one- threw it in jungle mixi-- a ice product sold at pet stores for lining reptile cages, mostly a light peat-ish mix and she came back strong. Brought her in for the winter on October.1 and it's throwing new shoots like crazy!

I bought 2 Japanese Fastias at a local Nursery... I planted them the same day in exactly the same soul type... one is thriving wonderfully and one started drooping a couple of days after planing and now is completely dead... any ideas on what could've happened? They were both beautiful, full, hardy plants when I bought them... !!!???