There's more to the Vamp than just an amp.
(1) It's DAC is a much better source than the iPhone headphone out, and (2) it acts as a digital dock to connect your iPhone digital output into your high-end rig via optical out. Some people spend big bucks on a digital dock, so they don't have to power up their computer to get lossless digital music into their audio system.
I bought one for my son for Christmas, and it's very nice, although I prefer my HiFi-M8. I also use mine as a desktop replacement with balanced full-size headphones, so I don't mind the thicker size of the M8.