My worst nightmare.
Say yourself a load of hassle and buy new rad, cost less than €70 want a go at rremoving vent try this.
1 Turn off valves both side of rad
2 rip rad valve and drain rad into bucket
or
3 rip each valve together, place thumb over rad end , lift and spin upside down, I.E rad full of water but water not allowed to spill(due to thumb cover) A two man job and how plumbers do it.
4 After emptying rad, lay rad on flat surface, get metal drill slightly smaller than vent and drill slowly.
I have had mixed results with this, depends on rad condition and age. It's a long time since I tried it, easy to replace rad.