The Vise Channel Blocks -
In that picture I used HDPE that was 1/2" thick and 28.65mm wide to fit the Vise channel as tightly as possible.
The correct dimension for the blocks is "Snug"
The lower in the vise the better, so thinner material would be better, 3/8" is good.
Regarding the vise jaws pushing against the stock or the blocks-
Ive tried it both ways and it depends on the job; the thickness of the material; the amount of material being removed.
When you remove a lot of material, the stock will bend in between the jaws and the work can shift.
The reason why the flip frame works so well is because of its frame structure, so tryto keep a frame around your material, the hammer handle for example:
I would use the stock to clamp against to start and not the blocks.