I just purchased a VLB (Vinyl Leather Brush) from The Rag Company. I have to say that this is absolutely the best tool for cleaning leather! It really gets deep into the grain and loosens/removes even old ground-in dirt that microfiber towels just won't touch. I used it in conjunction with Leather Master car interior cleaner (strong) and got excellent results. I have also used the VLB to clean shoes and leather furniture with equally impressive results.
One thing I've noticed while searching the internet for the VLB is that you can't really get it anywhere else but The Rag Company. If there's a way for Massdrop to source it, this would be a great item to have on a Drop.