Is kissing a person on the lips other than one's spouse cheating? What about not
In answer to your first question ("Is kissing a person on the lips other than one's spouse cheating?"), the very idea of "cheating" (conceptually) involves breaking a rule or agreement or promise, and so kissing someone other than one's spouse on the lips would be cheating if you had an agreement (explicit or implicit) that one would only kiss one's spouse on the lips, just as you would be cheating if you cried or laughed or sung a particular song with another person if you had promised only to do so with one's partner / spouse.
Before moving to your suggestion about promises, a brief note: I am a little curious about the example you give of kissing as there are many cultures (I have no idea how many) when kissing another person (who is not one's spouse) on the lips is not at all unusual or thought to be even remotely sexual (and thus a domain in which sexual fidelity would not be an issue). Actually, in the first two centuries of Christianity in Europe, unmarried men and women would regularly kiss on the lips during religious services in what was called "the exchange of the peace" though apparently this practice eventually needed some regulation for by the third century you can find precepts that the kissing should not involve open mouths and nor should the kissing be repeated multiple times during the same service! Thinking about this a bit more....I believe that (in general) it is quite common in many cultures for adults to kiss their children and other relatives and good friends on the lips without there being any implications about sexuality and thus not a matter of sexual fidelity.
Over to your suggestion about vows..... Your example of a vow or promise is: "I love you so long as you fulfill and do such and such...conditions according to MY needs of such and such..." As I acknowledged at the outset, these are private matters and there is no official philosophical policing of vows.... Still, I suggest three things are worth considering (and these might be more of a reflection of my personal dispositions rather than reflecting "objective values"): first, in entering a committed relationship though the testimony of "I love you" I suggest there is a kind of self-offeirng. Your are giving yourself, and your needs... over to the other, and he / she is giving him or herself over to you. So the promise is not, first and foremost, about "MY" needs as "OUR LIFE" together. Second, while it seems natural and expected that a marriage or relationship commitment would be conditioned on "the other" (it would hard for me to stay married to my spouse if she chose to divorce me and marry a lesbian), but loving each other is another matter. I hope my promise to love my spouse is unconditional even if she ceases to be my spouse and expressing my love for her becomes something entirely different than it is now. Third, over to promises and kissing...... If we set aside the many conditions in which kissing a non-spouse on the lips is permissible and expected in many circumstances .... let's let the focus be on the following:in a relationship in which one has promised to be faithful sexually with one partner, but the promise has been a bit vague (for example, there is no fine print on whether it is unfaithful to hold hands with a person who might be a potential love-interest outside the primary relationship). Under those conditions, I suggest a faithful partner would err on the side of being extra careful that one's actions with others do not send the message of (or actually express) desiring "to cheat." Intimate trust is such an achievement that can take years to develop... why risk it by doing acts that might well be interpreted by one's partner as sending the message that one more committed to having one's own needs met even if this means severing a committed relationship rather than seeking to have your and her needs met in the relationship you both entered through the door of promising to love one another?