Hello
A RP address does need to be reachable so i am not sure what you mean by virtual?
As for directional pim, My understanding is it has the advantage regards the way MC is propagated to the receivers, instead of the First hop router sending the unicast packet to the RP for it to forward to the receivers, what BIDR PIim can do it directly send the MC to any receivers that are inpath between first hop router and RP thus it cuts down the time forwarding of MC can be established.
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Kind Regards
Paul