Nr Fairy,
Sorry to contradict but the higher the disc loading, the LESS likely is vortex ring to occur for a given rate of descent, not more likely.
Vortex ring occurs when the velocity of the flow caused by vertical descent approaches the rotor down wash flow velocity. It allows a "doughnut" shaped vortex to encompass the outer part of the rotor disc reducing the effective angle of attack of the blades.
So bigger, beefier and heavier aircraft with a rapid / heavy downwash don't tend to suffer as badly as small, lightly loaded aircraft. Not to be confused with "running out of sufficient power to arrest a rate of descent" which has occurred before, not actually VRS.
However, the tilt rotor apparently needs very cautious handling at the low speed end, especially when rolling into a turn as the inner (down going) rotor, sees a higher effective ROD on that side. Coupled with an already high ROD, it might give rise (sink
) to VRS on that rotor, causing a loss of control.
Can't imagine one ever going into EGLW Heliport...