I finally have a workflow for getting my CardDAV contacts into #aerc. It's not pretty, but it works. I'd like to have used vdirsyncer for syncing up my Zoho vcard contacts, but it keeps bombing out on the import. So I had to give up on that.
Instead, whenever I have updates on the Zoho side that I want to get into aerc, I manually export from the web interface of Zoho my contacts. Then I run a Python script I wrote that will convert that file into a directory with vcf files that #khard likes working with. I have aerc configured to use khard to do the search and return results for aerc to display as I compose my email.
It's a pretty easy process even though it's not perfect.
Thanks go to @amin for providing pointers and tools!