I think what most folks do, and what I did, is to take off mirrors, thoroughly scrub with alcohol.  Mask appropriately.  Then use black "Satin Black" spray paint.  I used the stuff from Ace Hardware; works great.  Don't use "Gloss" or "Flat".
You will be very happy with the result.
And...can do the same treatment on upper and lower front grills, and also air intake louvers at the rear.  Makes things look really nice, and if you REALLY clean plastic before applying paint, lasts for years and years.  Like over 10 years for me; but I admit I keep my van in garage.

No...you don't have to use paint "for plastics".  You can if you want.  But good ole regular Satin Black Spray Paint, in several very light coats, works just fine.
Don't try Armor-All and all that stuff; after this many years it's too late.  Will only make it hard or impossible for paint to adhere.

RichSan Diego

      From: Gilles H Turmel <gilles.turmel@SYMPATICO.CA>
 To: vanagon@GERRY.VANAGON.COM 
 Sent: Sunday, July 10, 2016 2:55 PM
 Subject: Paint for mirror
   
Hi,
I’m looking for the best way to get my mirrors black again. Any suggestions?
Best regards.

Salutations 
Gilles Turmel