This is very likely not the full story, unless the 0day in VSA was somehow wormable. That "deployment" is doable through overly permissive IAM and everything else that enables privesc.
There are two parts to these vulns. Whatever gets the foothold, and whatever allows privilege escalation. Audits do a great job in catching the misconfigs that allow privesc.
The tragic thing about these attacks is often the blast radius can be contained fairly easily by asking the right questions... If you're someone who has passed these audits, or done these audits, it becomes pretty easy to see how many unforced errors go into these catastrophic attacks.
Without seeing the codebase in question, you can't be sure, but having been a web app pentester for 10+ years, these are the kind of issues that were found regularly, and whenever I saw classic ASP in tests, they were the kind of issues I'd be looking for, knowing the inherent weaknesses in the platform.