How to disassemble this v brake arm?
Looks to me like you've managed to unthread the brake boss from the frame.  That's not part of the brake lever.

Normally you'd undo the dome-head silver bolt visible on the left side of your photos.

If you stick a 5mm allen key/hex tool in that bolt, and mount the allen key in a vise would be a start.  Then you need to grasp the thread and undo it without damaging the thread.... that's difficult.

One "dirty hack" is to find two nuts of the same thread and internal diameter.  Thread them on fully and cinch them up against each other as hard as you possibly can.  Then put those two nuts in your bench vise tightly, and use a long arm 5mm hex tool to lever it off.