When I bake the vertex paint it does not create a backup layer and the texture is not bake
Curious, I’m also testing around with medieval shields right now. I am working on the bend.
Are you sure you don’t already have a TEXTURE on the UV before baking? Test in the Light menu - Texture ON / OFF.
Because: after baking your object is - white - that means he has baked white color.
And if white color is baked, then no backup layer is created - according to Stephan.
I made this shield because I saw your video and that’s where I discovered the bug. I will make a video to reproduce the bug
Solve … In fact, I didn’t notice that I was no longer in vertex paint when I did a bake again… it would take something that would make it easy to know if you are in vertex paint or texture.
The same thing happened to me and of course Stephan pointed out my mistake 
.
If you know, then it will not happen again.
I check the use texture switch in the light menu.
Note that vertex color is always active (multiplied against texture), that’s why it needs to be backup in a disabled layer.
Roughness and metalness are ignored when a texture is present, so it’s kept intact.