I think I am confused.
Does anyone Vorlauf through the sparge arm? or are we talking about recirculation through a sparge arm? and then change hoses around for actual fly sparging?
I don't use a sparge arm at the moment, after I made my stand and went to three keggles I recirculate the MT with the flow rate throttled back, when I stir it I always get grain through the pump and it clears as the grain bed settles a little. Prior to batch sparging I give it a good stir, wait for the flow to clear of all grain (vorlauf) and then move hose to BK.
When I did fly sparge I still didn't use a sparge arm from the HLT and when I vorlauf the MT I just did it into a container and poured it back into the MT slowly to not disturb the grain bed. All that was prior to fly sparging.