Shulchan Arukh, Yoreh De'ah 211/שולחן ערוך·יורה דעה רי״א

1One who states: "The vows I wish to vow shall not be vows", and then vows - this is no vow. This is only the case when one expresses the condition verbally, but if one merely thought of this, it is a matter of the heart, and cannot cancel the vow that was spoken. There are those who say that even if one whispers such a condition, it is like a mater of the heart. RAMA: As for us, when we say Kol Nidrei on the eve of Yom Kippur, it is as if we are making this condition. Nonetheless, we do not rely on this to permit things without asking the rabbis, except for urgent cases.

2One who makes the condition: "All vows that I vow until the following time shall be void," and then vows within this time - if they remember the condition at the time of making the vow, then the vow is valid, since the condition is annulled by the vow. If they did not remember the condition at the time of the vow, the condition remains valid and the vow is annulled. There are those who say that the condition cannot annul the vow unless one mentions quickly (toch k'dei dibbur) after the vow that it is being done according to the condition - and one should heed to this opinion.

4All this applies only to vows and oaths one makes alone. However, for a vow or an oath concerning someone else - these conditions make no difference.