Vain

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Middle English (in the sense ‘devoid of real worth’): via Old French from Latin vanus ‘empty, without substance’.

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From Middle English veyn, from Old French vain, from Latin vānus(“empty”).

etymonline

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vain (adj.)

c. 1300, "devoid of real value, idle, unprofitable," from Old French vain, vein "worthless, void, invalid, feeble; conceited" (12c.), from Latin vanus "empty, void," figuratively "idle, fruitless," from PIE *wano-, suffixed form of root *eue- "to leave, abandon, give out."

Meaning "conceited, elated with a high opinion of oneself" first recorded 1690s in English; earlier "silly, idle, foolish" (late 14c.). Phrase in vain "to no effect" (c. 1300, after Latin in vanum) preserves the original sense. Related: Vainly; vainness. Compare also vainglory.