Acts as a methyl-end fatty acyl coenzyme A (CoA) desaturase that introduces a cis double bond between the preexisting double bond and the terminal methyl group of the fatty acyl chain. Desaturates (11E)-octadecenoate (trans-vaccenoate) at carbon 13 to generate (11E,13Z)-octadecadienoate, likely participating in the biohydrogenation pathway of linoleic acid (LA) (18:2n-6).