Hey guys- news flash, the standard war trophy of Vietnam was the famous SKS, the Russians' answer to the M1 Garand.
It was developed before the creation of the AK, and continued to be used by the Soviets in their military aid program for fledgling Communist nations. The SKS was particularly prized by US soldiers in Vietnam because it could be sent home unaltered. Here's a charming story about how lots of veterans got their trophies.
It's likely that Kerry's war trophy is a SKS, if it's unaltered.
Update: Kerry says that the weapon isn't an assault rifle, but a bolt-action rifle. This would make it a Mosin Nagant clone, most likely. That's actually a lot more interesting.