Use 'vacillate' to describe someone who cannot make a firm decision and changes their mind frequently. Every time we discuss travel plans, Mia vacillates about the destination.
Pro Tip 2/3
Emotional Context
'Vacillate' often implies anxiety or stress about making a decision or choosing sides. Under the pressure of the moment, he vacillated, unable to choose.
Pro Tip 3/3
Not Physical
When using 'vacillate' about decisions, it's metaphorical, not describing physical movement. She vacillated between studying law or medicine, but never left her chair.