HI Pansyflower,
You should be able to do it with the little tool that goes on the bleed nut,,if this is not possible try lifting the rad off it holders,there should be enough movement to do this and accrss it this way.
If you use a vice and it slips you could damage the nut so that you won't be able to grip it. Give the nut a good soaking in WD40,leave for a while, soak again if necessary and then use the correct tool for the job. If your rad is that close to the wall that you can't access the bleed then it's too close to the wall. You should consider re-hanging it.
Go to local plumbing store and buy different types of VENT KEY'S. Hopefully you will get one to fit the damaged vent. It can be a pain as no vice grips will do the job, (trust me, I have tried!).
I normally have to replace the rad if the vent is badly damaged. I have tried drilling out old vent and re tapping another vent in it's place, time consuming and not always succesfull.