Trend: Vests. I love the few vests I own! I generally wait till something goes out of style to wear it, but vest are so cute!
Dress It Up: To dress up a vest (in my opinion-IMO) you should wear it over semi tight jeans, heels (boots would look really cool), and a V cut shirt.
Dress It Down: IMO you should wear it with jeans and over a fitted tee shirt. If you wear it over a baggy tee, it might look a little odd. Also, you could wear it with a lace tank top.