Vacillate comes from the Latin vacillatus, the past participle of the verb vacillare, ‘to sway or waver’.    

The earliest known use of "vacillate" in English is from the late 1500s, specifically in 1590s, in the sense of ‘swaying unsteadily’. 

Examples of vacillate in a sentence:
As we vacillate and procrastinate here, we are suffering.
Not knowing what vocation to choose after graduating, he vacillated among business, the ministry, medicine, and law.
He vacillates between different perceptions of his life. 
Jason shook his head, astonished by his own ability to vacillate.  

As my children were growing up, their behavior often caused me to vacillate between which one I liked the most. 

58 minutes ago, Bill W said:

Vacillate comes from the Latin vacillatus, the past participle of the verb vacillare, ‘to sway or waver’.   

Here we have another example of a Latin verb hanging around into modern Italian where vacillare still means to sway or wobble.

Whenever I need to use effect or affect I pause and monetarily vacillate to ensure I use the correct one.