Going to a smaller tire would help but first adjust your Mono shock pre-load, on your Mono shock there is a collar with I think 7 load level adjustments, riding 2 up requires you to go to the highest preload setting #7, also if you have lowered you bike there is a snubber called "rear shock pre-load improvement kit" that can be installed to limit the travel of the rear suspension, I did this on my bike and it did help, unfortunately lowering a bike makes it a personal adjustment and the down side is that you have not altered the suspension travel and therefore lost the ability for 2 up riding if both passengers and luggage exceed 200 to 225LBS or more depends on how it was lowered, how much it was lowered and condition of rear spring and suspension, mine no longer rubs but does bottom out on the snubber on large bumps, I am 190LBS and I ride solo all the time. Here is a PDF for lowering the Vulcan 800 pay attention to the plastic snubber they install in the bottom to limit shock travel, I made my own with some old 1/2 or 3/8 " plastic cutting boars and silicone to hold it in place, works OK and was free.
Lowering the rear end of the Kawasaki Vulcan VN800 Classic Low Rider
read this thread "Lowered VN800B rear tire rubbing" a search would have easily found it:
2002 VN800B, 6" DYI forward controls, EPA system removed, baffles removed, DYI lowered 2 inches in rear , 2" riser, mustang solo seat, re-geared to 18T/42T, scooterized and re-jetted carb, chrome rear fender rack, led headlight, old school leather saddlebags, billet chromed foot pegs, ISO hand grips
Last edited by jfl1960; 09-09-2016 at 03:52 PM.