What do you mean by that? Are you talking about a (rare) “cow rift” (it’s a nephalem rift that happens to be all-cows)? If so, then you surely are missing something - Vision portals do not spawn in rifts. If you are, however, talking about the Cow Level (opened in the wild by a goblin, or by using a Bovine Bardiche in the cube), and not getting a Vision portal inside it, then yeah, unlucky.