Yes, if a DB maintenance job runs it'll clean up old events from the database. So it's not really a PyVmomi or SDK issue, but just the fact the actual event is cleaned from the DB.
The disk creation time stamp isn't a bad idea, that's creative. I guess the negative there is you won't be able to get who created the VM, just the creation time.
If you have control of your VM creation process, you can add some CustomValue tags in vSphere to the VM for the creation date. I had a few customers who used a daily script or vCO workflow to get the VMCreated events, turn those into CustomValues (so they don't get pruned on a DB maintenance job) and that works well (since it's rare that you'll have VM events wiped in the 24 hr period of the script/workflow task).