That 16 gauge two wire AC cord included with the VTA ST-70 and ST-120 amp kits is 13 amp rated. Since the fuse blows at 3 amps on the ST-70 and 5 amps on the ST-120, the included cord is quite adequate for its intended purpose.
The original ST-70 came with a two wire cord. You can, if you want, replace the two wire cord with a 3 wire cord and ground the third wire to the chassis. The only warning here is that if any other component in your music system also has a 3 wire cord, you could set up a "ground loop" and the hum level could increase. You could try the three wire cord and note if there is any increase in hum level. If there is, just remove the third wire inside the amp. A shielded AC power cord is not necessary and will have no influence on the amount of hum/noise that the amp may have.
As a side note > The VTA M-125 monoblock amps do use a detachable 14 gauge 3 wire cord rated at 15 amps.