There are quite a few variations of the Vivitar 28mm f2.5, and from different OEMs for most mounts. The common one is the Kiron version (serial starts with 22) with a 62mm filter size. The one I have is all metal and quite large for a 28mm lens. It's half a stop faster than the f2.8 version. The lens is very well made and focuses very smoothly.
Art -- NEX-5N & Vivitar 28mm f2.5 [MD]. Click for larger.
The lens is a bit soft at f2.5, but sharpens up nicely after f4. I find it very enjoyable to use, as the focusing ring is very large and easy to turn. Definitely a consideration if you are looking for a mini-wide angle lens that doesn't cost you a lot of money.
Clocks at Distillery District -- NEX-5N & Vivitar 28mm f2.5 [MD]