Those ones are pathetic, putting an anvil on them when they can't take it! I have split an anvil vice using a hammer on a smaller one than this. It was like aero chocolate inside.
You know that you could quite easily replace that broken section with some heavy walled SHS/RHS or else some C beam and if it had too much gap in between the walls you cold just pad it out with some stitched flat bar, the front could easily be fabricated to hold the front jaw as well.