while there are many reasons for PPH, the most common is actually "cord traction." it's a process where by the midwife or doctor pulls on the cord to "release" the placenta. this usually tears the placenta from the wall of the uterus, causing a hemorrhage, whereas letting it detach on it's own usually does not cause a hemorrhage. there are, of course, other causes, but this is a pretty common practice, and thereby a pretty common cause.
so, many women experience PPH, regardless of diet and lifestyle and whatever else, for this reason.
as for the 'look' of the placenta, i can't say. i have really only experienced my own first hand, and it was awesome. i was not vegan, though. so, who can say? i suppsoe tehre are pictures of placentas on line, and perhaps a number of them from vegans, but i can't say.
i've never "heard" of it being a problem, nor seen any literature on the matter (scientific or otherwise).