Hi Tim,
Vitellogenins are the genes used for bulk yolk transfer in egg-laying organisms. The minuscule amount of yolk found in placentals is not VIT based. So, there is no reason, from a antievolutionary perspective, to find VIT sequences in placental mammals, since placentals have no need for bulk yolk transfer. Evolution, however, predicts that placental mammals are descended from egg-laying ancestors, and thus may retain fragments of the VIT genes harkening back to that former way of life. If so, these fragments should be in blocks of syntenty conserved with egg-laying vertebrates. So, what we observe in placentals is exactly what evolution would predict.