The Soviet Union didn’t just vanish without a trace. The process of dissolution was complex and involved agreements among internal and international organizations. Russia was recognized under international law as the legal “successor” to the Soviet Union. That’s why Russia assumed the former Soviet seat as a permanent member of the UN Security Council. It’s also why Russia took on the responsibility for the Soviet Union’s debt obligations to other countries.
I’m aware of those points and the situation was much more complicated between Ukraine and Russia and nuclear weapons than Florida seceding from the United States.