UPDATE: Turns out I just need to delete the computer that was already in active directory to get it to connect again. Sorry, I'm still learning my way around vCenter. It's connected again, but the original problem still persists. Disconnecting and reconnecting did not fix the issue.
Oof,
I tried RajeevVCP4's solution first. After removing my vCenter from the domain and restarting it it can no longer find the domain.
The cli give me the error message "Error: NERR_DCNotFound [code 0x00000995]" and the web client gives me the error "ldm client exception: Error trying to join AD, error code [2453], user [DomainName], domain [domain.com] orgUnit[]"
Now, I've made sure I've opened ports 123, 135, 137, 139, 3268, 389, 445, 464, 88 using both tcp and udp as per this guide (https://www.altaro.com/vmware/how-to-join-esxi-to-active-directory-for-improved-management-and-security/ ) , I've created a host record and a ptr record in my DNS using the ip address of the vCenter, I've configured my vCenter to use my DC as an NTP service so their times are synced, but according to my firewall logs the vCenter server makes one query to my DC using port 53 which I've read is the DNS port and then fails.
I'll continue to search those logs that Gidrakos mentioned, and if it matters the answer to their first question is that this was an upgrade from an older version of vCenter (5.5 I believe), but was rebuilt a few months ago because it wasn't working correctly. Any other ideas welcomed and appreciated.
Jared Keyes