I searched all around Velvac's site and the internet, but didn't find anything about the arms (just the heads). Of course it's one of those things that there are a million ways to say, so not that easy to search. Also many RV's have the Velvac 2020 model (including some Chinooks), so much of the information I found pertained to them (and the arm/base interface is not the same on the 2015 and 2020 models).
Eventually I called Velvac, and got the info I needed to adjust the arms, so I thought I would post it here for reference.
First of all, to see if you have the 2015 model, look at the way the arm joins the base at the van body. On the 2015's, the arm sits in a U-shaped cup, with the base of the U facing forward, as in the photo below. The 2020 does not have the U-shaped cup.
On the newer style, the screw can be found on the rear-facing side of the long arm just where it meets the base (i.e. close to the body of the van, and where you can see it while in the driver's seat). On the older style, from what I understand, you need to "fold" the mirror arm in and then you can get to the screw (so it is hidden while driving).
I've now driven a couple thousand miles since I adjusted mine, and it totally did the trick. No more "creeping" mirrors, even in headwinds or on bumpy roads. I'm glad because my mirrors are in good shape, however, as the 2015 is no longer made, certain parts aren't available (it's then suggested that one replace the whole shebang with the 2020 model; also nice mirrors but expensive).
