Dave, I don't know about other Long-EZ'z. I was very careful when I instaled my canard after reading CP 35 page 2, 01/1983. In short it said the Long's with the most pitch down in rain was caused by not fitting the Canard per plans. I used a transit to make sure. I guess it worked cause I had little or no pitch down in rain without VG's. I put the VG's on to get more pitch athority at slower landing speeds. They do that. Another thing I changed that I feel helps is the Strong electric pitch trim. It so much better than the per plans system.
In short I can't tell you why they work. They just do. There are drawbacks with VG's. First, you can't sit on the Canard anymore. You can't keep people from touching them.