Ah, you mean horizontal and vertical together. At first I thought you meant 2 different methods.
I think we talked about this before, but wasn't a big deal since most browsers will scroll the menu list and clicking on menu header takes you to brand listing and index page. I had the same concern until I figured that out, now it's a little annoying, but works. Try chrome if this doesn't work in your browser.
I do think it is getting a bit unwieldy as more brands are added and just having a simple index list would be easier. Knowing to click the menu header isn't obvious either.
If you can do a grid (horizontal and vertical) drop down menu that would work. Otherwise adding a first option of [brand index] as the first option would help. This could go to existing index page, but having a horizontal and vertical list of anchor links at the top would be ideal to minimize scrolling past all the images.