It shows that the vain oath here means that which has no...

It shows that the vain oath here means that which has no effect on the intention of the speaker, that is, such oaths which one utters (like 'No, By God', 'Yes, By God') without taking those words seriously. "al-Kasb" (means to earn profits by a work or profession etc. Originally, it was used for the obtaining of those things which fulfill material needs.

Then it was metaphorically used for any good or evil a man may get as a result of any of his actions, like earning praise and good, reputation through good character and social services, and earning good knowledge, superiority, and nobility by striving for them; or earning condem­nation, abuse, and slander; or sin and error by one's evil actions.

This is the meaning of al-kasb and al-iktisab Some people say that al-iktisab is used when one earns a benefit for one's own self; and al-kasb refers to earning a benefit whether it is done for one's own self or for others, as when a servant earns for his master, or a guardian for his ward. In any case the active participle al-kasib and al-muktasib (both of which mean "one who earns") are used only for a human being. Anyhow, now we come back to the verse.

The sentence, "but He will call you to account for what your hearts have earned", is a metaphor in a metaphor. The preceding sentence says: Allah will not call you to account for what is vain in your oaths. The contrasting sentence should have been, ". . . for what is firmly considered of your oath". Instead it mentions the effect, that is, the sin, which will come into being if one breaks that vow. It is done to show that Allah looks only at the heart, as He says:.. .

and whether you manifest what is in your souls or hide it Allah will call you to account for it (2:284); There does not reach Allah their flesh nor their blood, but to Him reaches your piety . . . (22:37) The words, "and Allah is Forgiving, Forbearing", hint at the undesirability of vain swearing and oath-taking, because such a thing should not be done by a believer. Allah says: Successful indeed are the believers, who are humble in their prayers, and who keep aloof from what is vain . . .

(23:1-3) QUR'AN: For those who swear (to abstain) from their wives. . . surely Hearing, Knowing: 'al-Ila' " (= to swear), in Islamic jurisprudence, means the swearing by husband that he will not go to his wife, provided it is done in anger with an intention to harm the wife. This is the meaning intended in this verse.