A quick follow-up to say that I was able to use Layer > Create Compound and then Layer > Convert to Curves to simplify complex images, then add the resulting vectors to the Asset list. For some layered items, however, compound images merge background and foreground fills into one color, so a banner with black outlines and white fill turns all black, for example. I'm getting around that by first going into the layer list and finding the background layer, turning it off, then using Layer > Create Compound and Layer > Convert to Curves on everything but the background, then repeating the process for the background layer only. This results in two vector assets which can be aligned and colored separately.
It's possible I'm the only one who didn't know this trick, but as my searches didn't find it I'm posting in case it helps someone else!