Many of the new discount Asian Import vises are made of cast iron.
Get a good American made vise... maybe 30 years old, and you'll have a heck of a time breaking it.
What are you breaking on the vises? I've seen broken mounts or swivels, but I've never seen broken jaws, I don't think.
Hmmm...
Look for a Wilton brand Vise.
Not cheap, but they come with a lifetime warranty, and they mean it.
Wilton Tools | Home