I have painted several pulling tractors, and have done this many times. I have even sprayed clear coat over a partial vinyl wrap. I have not had any issues with any of them. At first I would put the decal directly over the base coat, then clear coat it. But on the last few due to the small margin for error, when there is no clear coat, I have been spraying a couple coats of clear, then wet sanding with 1500, before the decal. Then spray a couple more coats of clear, then cut and buff.
The other problem with putting decals directly over the base, without clear, is if there is a dust particle in the base, the decal may have a bump in it that will possibly get sanded through when cutting and buffing.