Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
vānĭtūdo, ĭnis, f. [vanus], emptiness, nothingness, vainglory, vanity (ante-class.): ne turpasse vanitudine aetatem suam, Pac. ap. Non. 184, 7: vera vanitudine convincere, by empty, lying talk, Plaut. Capt. 3, 4, 37.