wow, 3 version of your post now.
as one person said ..
silicone.
my thought it dust getting in there, or oil fumes coming out ..
I lightly seal it to the case with silicone. 
doesn't seem to hurt though that most of them are running around with the tube stuck in the hole and that's it. There's an o-ring there maybe, as well. 
  ----- Original Message ----- 
  From: Gregory Smith 
  To: vanagon@GERRY.VANAGON.COM 
  Sent: Thursday, July 29, 2010 7:33 PM
  Subject: Dipstick tube loose, how to fix?

  Noticed my dipstick tube loose at the case, obvious potential vacuum leak.  I n the past I've just cleaned the area as best I can and dabbed some JB Weld about the tube base.  Any other suggestions?

  Thanks,
  Gregory Smith

  '88 Vanagon GL

  Smith Design Studio
  Small Building Company
  Olympia, WA
  360-239-0429
  gregoryfsmith.com