What type of Vesa do you have?
The Vesa should split into two parts. The wall bracket and the screen mount. Fix the wall bracket to the wall, attach the screen mount to the screen, hang the screen mount on the wall bracket.
I think the screen mount is part of the stand of the pc. It looks right and it is used to afix the pc to the stand. I can remove it from the stand but it is still in a plastic casing and I can't get it out.