Why was Vietnam a significant turning point in the Cold War?

The Vietnam War enabled the USSR/Red China/US/UK/France/Israel/South Korea/Etc. see the effects of and combat test all of the world's "cold war" weapons...from the B52 to the M16 assault rifle, from the M48 Patton tank to the new aluminum airborne M551 Sheridan tank. From the Ryan UAV Firebee to the SR-71 Blackbird Reconnaissance airplane.