Your view of history is far from accurate. The "vote" by crimea to leave ukraine was after Russia took over Ukraine in a military action. Voting to secede after someone takes you over and that Russian someone doesn't do democratic voting.
"The Crimean status referendum of 2014 was a disputed referendum[1][2] on March 16, 2014, concerning the status of Crimea that was conducted in the Autonomous Republic of Crimea and the city of Sevastopol (both subdivisions of Ukraine) after Russian forces seized control of Crimea.[3]"
From Wikipedia - https://en.wikipedia.org/wiki/2014_Crimean_status_referendum
"The Crimean status referendum of 2014 was a disputed referendum[1][2] on March 16, 2014, concerning the status of Crimea that was conducted in the Autonomous Republic of Crimea and the city of Sevastopol (both subdivisions of Ukraine) after Russian forces seized control of Crimea.[3]"