That will NOT be able to fly.
The whole cause of Vietnam War is to see if it's "two Vietnam's" (USA point of view) or "one Vietnam" (Vietnamese point of view). RVN may be able to pull that off (asking for more money from USA to appease the populace, or at least the elite ones) - though it is still risky. DRVN will never, ever, do that, because it would go against their very declaration of independence, sworn during the 30s and 40s. The only type of "Korean War-style armistice" that is acceptable would be an armistice followed by a General Election.
You know, the one in Geneva Accord in 1954 but the US refused to hold it because they knew the Viet Minh (and by extension, the communist party) would win with a landslide?