What is the ruling on a former smoker who vowed and swore not to return to smoking, but then desires to smoke a single cigarette on some occasions?

You must fulfill your vow and oath and not return to smoking. If you do, you will be sinful in two ways: failing to fulfill the vow, and falling into disobedience intentionally. Similarly, it is not permissible to deliberately break an oath if you swore to abandon a sin. You should also beware of following the footsteps of Satan, who might corrupt your repentance and drag you into other sins, as Allah Almighty says: "O you who have believed, do not follow the footsteps of Satan. And whoever follows the footsteps of Satan - indeed, he enjoins immorality and wrongdoing." So, preserve Allah's blessing upon you through repentance, and remember the reward of one who feared the standing before his Lord and restrained his soul from evil desires, as Allah Almighty says: "But as for he who feared the standing before his Lord and prevented the soul from [unlawful] desire, then indeed, Paradise will be [his] refuge."