Spraying plus gas on the back of the freewheel so seeps on the threads and leaving for a while before trying the vice again has helped before. If that fails, spray again and leave longer.
Also when using an extension and more levergae you run the risk of the tool slipping out and or rounding of the splines. If you have a Q/R axles then clamp the tool in place first using the Q/R and a few washers and then use the longer lever. To save using a long bar take it down a garage and put a 1" socket on your freewheel tool (if it a park or similar) and then get an air gun on it. That will provide all the torque you need.
I had to admit defeat with a BB this week as even with a long breaker bar and all my strength it would not shift (in excess of 500NM of torque was applied). I had been applying copious ammounts of plus gas as well. The air gun did it in seconds. Sometimes some things are really really stuck. The threads somehow where fine after removal and the tap span in cutting no metal, why it was so stuck I can't figure.
Try the plus gas first though as extra leverage and airguns are weapons of last resort. As if it really stuck applying all the leverage in the world could just mean you round the splines off and then you are really stuck.