I've read again about what's considered vowing, and saw a sentence in Ecclesiastes 5:2: "Do not be quick with your mouth, do not be hasty in your heart to utter anything before God." Does the heart part mean after all, that our thoughts about vowing are taken as vows after all?
Thank you again for your response!
"Hasty in your heart" means to be impulsive. The warning is don't make vows to God because you weren't thinking about what was coming out of your mouth or you only gave brief thought to it. The vows discussed are those said out loud before witnesses. But before you let the words slip out, remember that you are promising something before the Creator and Judge of the Universe. If you have an impulse to make a vow (a thought of vowing), then reign it in and give it careful thought because you want to be able to keep your word.
It is the words you say, not the thoughts, that create a vow.