Is a vow considered binding if one forgets whether it was uttered or if it was merely a thought?

It is true that a vow spoken to oneself is not binding, and one is not obligated to fulfill it. Its ruling appears to be similar to someone who doubts whether they made a vow or not, because the default is that no vow was made. So, if it is not certain that you uttered it, your vow is not binding. However, fulfilling it is more prudent.