If you are building a Paul Sellers bench, it will have an apron. you can make a leg vise work. Every vise has it’s strong points and weak points. A leg vise is more work to install, but it will work. Keep in mind Paul doesn’t use a leg vise. The leg vise can be less expensive, depending on the hardware you use and what you are comparing it too.
Your bench seems large. My bench is a Nicholson bench with a leg vise. 6 feet long 24 inches deep and 34 inches tall, I am 5’8″ tall. 40 inches tall is a very tall bench, unless you are over 6’6″. This video will help you.