If a major superpower falls, adversarial superpowers will be the ones to move in to fill the power vacuum, not local tribal warlords. See Russia after the fall of the USSR, but before Putin.
I don't think you can say there's a general "rule" like this, throughout history. If anything the cases where these powers become straight-up subjugated from the outside (absent outright military invasion) are in the minority. Counter-examples abound, e.g. the Ottoman Empire and Austria-Hungary, or Spain after the disaster of 1898.
See Russia after the fall of the USSR, but before Putin.
Post-Soviet Russia was weak for sure, but not a vassal state by any interpretation.
