Vintage vitra Eames 670/1 w/o label - discount?  

What's in a label?
If you are confident in your dealer and he offers a guarantee (he should), then proceed with negotiation. If you are buying for resale, lack of a label may effect the amount of time and effort involved, but not value, if it is authentic.
I've only ever seen one Vitra set of unknown vintage in the US. The bases were all black, if I recall, and the printed orange and white(?) label looked difficult to remove.

no reason required
ewe,
There is no reason required to ask for an "additional consideration" discount in every transaction (your chair).
When the idea of "future resale" is considered on an item with an "issue" I never purchase anything that I do not want to own and use (your chair). These also make good gifts (your chair).
When and if I sell an item that I owned and maintained (your chair), I feel lucky if I get my money back and I feel blessed should I make a profit.
I am not a beginner, I buy what I like and know. You must do what is best for you (your chair). Know what you want and like what you get (your chair). Fool proof.

Be really carfull!
I know you claimed that it is vintage. But I recently knew a person that bought one that was a "Vitra 80s model" that turned out to be a worn in reproduction from a few years ago.
There are a few company's that make Replicas or Fake Eames 670s/671s that are real good at getting all the details...
down to the spaces between wood...the curves, the base/height...you get it. So watch what you buy. And I also want a label when its an item of that price and something that is often replicated.
See this website Below. They sell a replica 670/671 that looks so close to Vitra. What they did (after speaking with someone from Manhattan designs) was simple...took the exact measurements of the HERMAN MILLER 670 and the Base style of VITRA so there would be no legal issues. If you didnt know,the HM version is a little more rounded then the VITRA one.
Check out this link
http://manhattanhomedesign.com/eames-lounge-chair-and-ottoman.html
http://manhattanhomedesign.com/eames-lounge-chair-and-ottoman.html

If that photo is Legit ^^
if the photo that you used up top is of the *actual chair your purchasing then Id say buy it. You can tell by the leather on the top panel that it is vintage. has that thick cow leather look which newer models dont have. Also you can tell a real eames simply by removing the cushions before buying it. Theres a thread somewhere on this site about identifying real/fakes.
But the one in the photo is certainly real.

Going back to your original question Uwe, I presume from your post you are happy with the chairs authenticity?
When it comes to a design like this a label means very little, once you poke about under the cushions and see how the chair is put together it is very easy to determine weather a chair is authentic or not.
Especially if it is a vintage example, you will always have the tarnishing to the polished metal areas, the patina to the leather and the smell! A vintage chair should have a certain smell to it, not unpleasant, if you have ever sat in a vintage car with lot of leather upholstery you will know what I mean.
If you are uncertain you know enough about the original design I would suggest you head for your nearest Vitra stockist and inspect one of their chairs thoroughly in order to make an informed decision.

H moons right>
If your really interested...a small amount of research and seeing some models in person will be enough for you to tell if this considered purchase is worthy.