Greetings 19,
These are side question, but what year is your VES valve? Does the threaded end screw into your guillotine slide?
Offered for your consideration.
Use a bench vise and squeeze the washer/shoulder end (looks like I can see some vise grip marks on the washer/shoulder ends in your picture) take your time you don't want to distort the shape of the washer/shoulder. If you have some metric nuts available you can double nut the threaded shaft and place in the bench vise. Have the nut looking up.
Find a piece of tin and lay over the top of your diaphragm to protect it. I've also taken a drill bit and drilled a hole in my piece of tin slightly large than the nut diameter to provide additional protection for the diaphragm. Use a torch and apply heat to the nut. Sometimes the diaphragm is a lost cause. Take your time it may take several attempts. I'm guessing there is some Loctite on that bad boy.
Good luck,
Craig.