You can't change the primary skill (left click) and secondary skill (right click) they are locked.
I think you can workaround the key binds and setup your skills like you want.
Let's say you have fan of knives on "1" and vault of left click.
Go to game settings / key binds and change 1 to left click, this way even if you have FOK on 1 and vault on left click, when you press left click you will use FOK and when you press 1 you will use vault.
Hope that helps you.
And even if you could it would mess up all the other characters and builds.