I struggle with much of the 'black magic' that is helicopter aerodynamics, and so attempt to look for the simple answers (which is not to ignore that a lot is occurring behind the scenes). Since the vibration associated with ETL can occur when on approach to an elevated helipad (i.e. when there is not yet any hard surface for the rotor wash to interact with) the vibration cannot simply be associated with the so-called 'ground vortex'. I have observed rotor wash in a still-air hover (e.g. over a body of water) and have seen visualisations that show the classic donut vortices evenly distributed around the rotor tip path. I have also observed the effect of rotor vortices in flight (both actually and virtually) where they resemble conventional fixed-wing tip vortices. At some point as a helicopter accelerates, the vortices that circulated around the entire tip path are forced to separate and migrate toward the extremities (the 'wing tips'). Noting that this separation and movement of vortices is quite violent, and that it is also likely to impact on the airflow over the tail rotor, it seems reasonable to me to assume that vibration will be marked. I'm happy to be corrected, but this is the simple explanation I give to my students.