You could convert any type of wheel driven spreader to VRT
I did it to a Willmar 8 ton spreader for P/K(had to go over ground twice for two products)
http://talk.newagtalk.com/forums/thread-view.asp?tid=34907&posts=3
Parts needed on the spreader
Hydraulic motor drive
Hydraulic control valve to control motor speed
Encoder to monitor apron speed