The velcro will likely hold as well with less mess, but total thickness of the stuff in that slot determines how hard it is to roll the window up.  The Weathertech deflectors have their edges milled very thin for this reason and I think this promotes breakage and slipping.
Stephen 

     On Friday, January 23, 2015 10:24 AM, Tom Stone <tstone8359@aol.com> wrote:
   

 I put an adhesive strip of Velcro "hooks" on the outside edge of mine.  This reduced the tendency of the deflectors to slide down when the windows were open and did not interfere with closing them.  Bought it at Home Depot or Lowes.  Hundreds of uses.  

Tom

-----Original Message-----
From: Stephen Grisanti <bike2vcu@YAHOO.COM>
To: vanagon <vanagon@GERRY.VANAGON.COM>
Sent: Fri, Jan 23, 2015 8:27 am
Subject: Re: [VANAGON] Which Wind/Rain Deflectors?

A dab of clear caulk or the like helps them stay put. Clips would be harder to 
configure. 

Sent.