I do Brew in a Bag for about a quarter of my brews. I've done big strong beers, light hoppy ones, quick sours, dark & smokies . . . all without a proper vorlauf. I've never had a clarity issue, tannic issue or storage issue (up to about 2.5 years thus far).
I've always assumed the vorlauf step was one of those kind of ridiculous things propagated through years of empiric experience. I still do a quick quart-or-so, but don't worry if I don't (say, if I'm trying to unstick a sparge).