Is using electronic cigarettes considered a breach of a vow to quit smoking?

The e-cigarette is considered a form of prohibited smoking, and abandoning it falls within your vow. Jurists have differed on whether a vow to abandon something prohibited is valid or not. The majority opinion is that it is not valid. However, it is more cautious to offer an expiation (kaffarah) for breaking an oath, and you are obligated to stop smoking in all its forms.