I (finally!) got myself a copy (the paperback edition) of Tom Shipley's book,
Man and Woman in Biblical Law. (
http://www.newcovenantpatriarchy.com/) He has an excellent exegesis of Leviticus 18:18. According to Brother Shipley (sure hope he's at the conference!) what would "vex" a wife would be if she is childless and you
take her blood sister as a wife in order to have kids. In a case like that (according to Shipley, and I agree), a man should look for a second wife (who will hopefully not be barren after marriage) among people not related to your first wife. (I'm paraphrasing Shipley's exegesis.)