Aluminium Group Vinyl vs Leather  

Nope, no stitching
Herman Miller's leather and vinyl may look pretty similar when new. You should still be able to tell the difference by the grain (more irregular on leather) and touch (leather feels warmer). Once they have a few years on them, it's a much easier, since they age differently. Leather will develop natural creases and shine where you sit. I also have a leather Aluminum Group chair from the 90s, and there's not question about the material when you see it.

Thanks
for the info.
Is one material preferred by the design fraternity? Which came first in terms of the chair's lineage, vinyl or leather?

Vinyl came first.
I happen to prefer leather.

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"Is one material preferred by the design fraternity?"
I cannot imagine vinyl would ever be preferred over leather.
Some crazy-ass purist wanting some early vinyl edition if it is
valued somehow as 'early/rare' may disagree. (it happens)
Personally i just don't like the cheap squeaky sound of synthetic resin.

leather vs vinyl
Leather and vinyl would have its own very distinct smell even the really good vinyl that looks like real leather would still have a distinct vinyl smell. If you can see the back of the material, it will also reveal that leather has that animal skin moccasin texture while vinyl will have a synthetic backing of some sort.
The Eames Management chairs designed for contract/office use was a spin off of the Eames Aluminum Group, originally designed for use as an indoor/outdoor residential furniture when it first debuted in 1958. The Eameses and Herman Miller realized early on that vinyl during that time and the aluminum frames do not fair very well when used outdoors.
If your chair is leather, it was probably ordered for an employee in a management level, if it is in vinyl and fabric, it was most likely for general staffing.
Most people prefer the status symbol that real leather brings (like mahogany furniture real or not) over vinyl even if it is the original upholstery. Like the famous upholstered furniture by Le Corbusier and Breuer during the 1920's, most prefer the leather versions which came in much later in the 1960's over the heavy cotton canvas soaked in oil material used in the originals (the 1920's version of a waterproof material still being used for convertible tops for cars before vinyl was invented).

Leather
In terms of wear and care leather requires a little bit more maintenance, but has a much longer life expectancy when properly cared for. Good leather will wear in with age...vinyl will eventually crack and split.
Also - leather generally commands a higher price, so if you do find out that your chairs are vinyl I would request a refund from the seller.