I have gotten very different results from the xbmts when I've not strained the husk materials properly. This has happened to me more than once. Maybe it doesn't happen all the time, but when it does, it comes through as a sharp astringency. Part of this could also be an alkalinity / mash pH problem. In those past batches, I probably wasn't as anal about pH as I am today (and still am not terribly anal, but am more now than the past).
If you really want to prove or disprove the notion of vorlauf preventing astringency, try this with a stout sometime. If you don't do that magical thing where the roasted malts are added only at the very end of the mash, and mash the whole grist together for an hour or whatever, and then don't bother to vorlauf at all....... you could very likely end up with astringency. Black malt husks are not kind when boiled in my experience. And you might think the opposite would be true, that a light color beer would have higher pH and end up with more astringency. And you might be right, maybe. But I do know I've had problems with doing a crappy vorlauf with black beers. So, anyway. I'm not convinced that this is never an issue. Still pays to vorlauf IMHO. I even do it for my BIAB beers. I don't want all that crap in my boil. I strain it out, whether it's necessary or not. It's so easy and not worth the risk not to IMHO.