Hi Chris; you may want to check out
www.leevalley.com. They have a twin screw vise that I've been using on the end of my bench for about 6 years now. Its a bit of work to retrofit it onto your bench, but it can handle a really big piece of lumber. Check it out. They have a FANTASTIC catalogue that they can send out to you. Its a company out of Ottawa Canada, so remember the prices are in Cdn. $ which is really good for you right now if you live in the U.S. Let us know if you find it, and how it works for you, it works great for me. Dave