Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
văcillātĭo, ōnis, f. [vacillo], a rocking to and fro, see-saw, a wavering, reeling motion: indecora in dextrum ac laevum latus, Quint. 11, 3, 128: foeda, Suet. Claud. 21 fin.