You are correct. The tractor should not be there. There are rules which allow workers on foot into the runway strip clear of the sealed surface under certain conditions, however not a tractor.
It can be hard to change entrenched rule breaking. I would approach your direct supervisor and keep going up the chain until you can get something done. Use outside agencies if it's not getting changed.
In the meantime, if the tractor is monitoring your frequency, tell him to vacate, if he's not, then use a recorded line to ask your gnd controller to vacate him. At least it's recorded.
If the tractor is still there I'd be using the phrase, 'cleared to land at pilots discretion' and make sure that notification of the tractor was on the ATIS or give traffic.
You might be surprised how many people agree with you if you start a discussion in the brew room. Good luck.