You could argue that during Louis XIV time, the Prince-Electors of Cologne followed a overtly pro-French stance.
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Maximilan of Bavaria during the Franco-Dutch war
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Joseph Clemens of Bavaria during the War of Spanish Succession
You could also have France winning the War of Palatinate Succession (also known as the 9 Years War), and have Elisabeth Charlotte (Liselotte) succeed her brother in the Palatinate, instead of the Electorate going to another branch of the family. She was married to Philippe, Louis XIV brother. This would mean that at her death in 1722, Philippe of Orléans would become Elector of the Palatinate. And at this time, he was also Regent of France! (he'd die in 1723).
If a French victory in the 9 years war doesn't have too many butterflies, than you might go on with a Bourbon-Orléans Elector of the Palatinate. I'm not sure the Emperor would strip them of the Electorate so long as the policy is not too pro-French (might be difficult to avoid though).
It's not a direct conquest, nor a real vassal relationship, but it's probably the closest France got to having an Elector in her pocket. If France conquers directly an electorate, then another one would be created, as it happened when France annexed the Rhineland during the Revolution.
I think Louis XIV or at most Louis XIII is when you might see an Elector falling under French influence.