I have vintage, pre reformulated and reformulated (last version) Guerlain Vetiver and vintage Givenchy's: the first one is more complex, the second is straightforward Vetiver. Don't get me wrong, it is a fine Vetiver, however on the same vein as Carven's. I even had the chance of trying Fueguia 1883 own rendition and it reminded me of the (the nose behid the House decided not to launch it for its hefty production price).
In other word, "Vetiver" as understood today are vetiver based scents that add other notes. The ones from the 1960's are just that, vetiver scents with all what vetiver offers.
From a good source in the industry, I have been told that viteveria zizanoides is harder to get each day because its harvest is very complicated. Thus workers choose easier tasks, like collecting oranges. So I think that prsent day "vetiver" scents are vetiverol based scents that make up for its lack of analogies with the natural oil through the inclusion of other notes. Just an hypothesis.
Just to wrap it up: I enjoy Guerlain's more than Givenchy's in all their renditions.