1) A vow does not take effect unless his words and intentions were parallel. This means to say that he must have intended the vow in his mind and also expressed it with his mouth. If, however, he vowed by mistake, in such a way that his intentions were not to make the vow which he uttered, and he made a mistake in his words, the vow does not take effect. (Kitzur Shulchan Aruch 67).

2)Similarly, if he only thought about a vow, and did not express it with his mouth, it is not a vow. (ibid)