To vacillate is to keep changing your mind, and to keep changing it back. It is not the same as changing your mind once because you found out something new — that is just being sensible. Vacillate means you go one way, then the other way, then the first way again, and you do not settle. People use it about decisions (she vacillated between the two job offers for weeks), about opinions (the government vacillated on the new rule), and sometimes about feelings (he vacillated between hope and worry). It is a formal word. You will meet it in books, news articles, and business writing far more than in talk between friends. In everyday speech people say "can't make up my mind", "keeps going back and forth", or "keeps flip-flopping". If you said "stop vacillating!" to a friend, it would sound like a joke, or like you were being a bit grand on purpose. It carries a small judgement. When someone says a person vacillates, they usually think that person is weak, or slow, or afraid to choose. It is rarely a compliment. The pattern is usually "vacillate between A and B", or "vacillate over/about something". Though actually — the original picture in the word is a physical one, something swaying or wobbling, not able to stand still. That older "wobble" meaning is basically gone from modern English; if you use it, people will hear only the mind-changing sense.

猶豫不決、搖擺不定、舉棋不定。指反覆變來變去、拿不定主意，通常帶有一點負面評價（覺得這個人不夠果斷）。