Since I both fly sparge in a 3-vessel, and BIAB in a single vessel, I can say with some confidence: I think you'll want to skip the vorlauf. The whole point of a vorlauf is to set the grain bed, which with BIAB you don't bother with, as you're lifting out the bag and messing up the grain bed. Yoopers and others who have a bag in their mash tun, could batch sparge or fly sparge, and would vorlauf, and may have a way to keep the bag off their outlet, but if you're boiling in the kettle you're mashing in, no need.