Hi, Leggs! I totally agree with Connie.
IMHO, you should NOT wear a weight vest on the treadmill regardless of whether you're walking or running on it. Not only for the joint stress due to increased impact that Connie mentioned, but because your center of gravity can shift because of the new weight load and this can alter your gait and balance - which can have some pretty mean consequences on a treadmill. Also, a weight vest tends to keep more heat in your core, and you run the risk of seriously overheating if you try to do too much.
IF you are already a skilled power walker, with good posture and a fast strong gait, a weight vest MAY have positive benefits as long as you start with a relatively light load (maybe 4-5 lbs), and this load is evenly distributed around your trunk and up your back. My suggestion, however, in addition to Connie's about exploring interval training, is to power walk in a hilly area - hills are great for increased intensity!
Just my $.02 -
Annette