Which is pretty much what I have posted already but without the use of the word 'envelope' since it can be misleading.
All we are talking about is rates of descent - something every pilot can easily relate to - big, heavily loaded aircraft won't encounter VRS until they have RoDs well in excess of 1000'/min whereas smaller, lighter helos can encounter VRS at RoDs well under 1000'/min.
Expanding the speed part of the 'envelope' is pretty pointless since you are unlikely to encounter VRS above ETL which doesn't really vary much with helicopter size.