Diorella was reformulated recently, and I am not sure exactly when, that is, I don't know if all Les Creations are of the new formulation or if some older Les Creations bottles are the unreformulated version. Perhaps somebody can shed light.
If you are not sure whether you smelled the recent or the old version, the recent version is primarily a citrusy and slightly green floral, pretty clean. It has been defined by perfume critic Luca Turin "greenish lemon tea". The vintage version was a chypre, that is had a deep mossy drydown, and smelled dirtyish rather than clean, with overripe fruit and likely some other dirty musks.
As you say, vintage Diorella is very difficult to find, and, when found, extremely expensive. If you know you smelled the recent version and you like it, go ahead and buy it, it's good on its own terms, and, as said, it's quite different from the old one, so you won't necessarily like the old one if you like the new one. In particular, you may not like the old if what you're looking for is a clean fragrance.