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It looks like I'll have to make my own.

I've never seen a tarragon vinegar that is GF either.  However, might you 
just consider a small pinch of dried tarragon for your dressing?  A VERY 
LITTLE bit of tarragon goes a long way, so I'd experiment with a very tiny bit, 
and then just add to taste.  I use dried tarragon all the time, just for 
this reason (no tarragon vinegar any more).  I hope you'll re-post to the list 
if you are told of a brand that is OK!
+++++
I use rice vinegar and add tarragon seasoning.  Easy!
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You can make your own GF tarragon vinegar.  A brief search with Google  I 
found this:
http://www.wikihow.com/Make-Tarragon-Vinegar
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I use Maitre Jacques Tarragon French Wine Vinegar that I find at Wegman's. 
Ingredients are: white wine vinegar, tarragon, potassium meta bisulfites as 
a preservative adjusted with water to .06 acidity. Imported from France. I 
have been using this with no problem.
++++++++

Reese tarragon sprig wine vinegar seems ok.
+++++

some people heat the vinegar or make your own
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The easiest thing is to make it yourself...just get some high quality    
fresh tarragon (widely  vailable) --make sure it is washed clean  (gently) and 
dried (sunlight is fine)...and pop it into a jar of high  quality vinegar.  
Wait a bit for it to diffuse it's flavor...

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