Clarification of "lose" is needed
There are several scenario for communists to "lose" in Vietnam in 1975.
1. North Vietnam was conquered by the South or/and US, or at least South Vietnamese army were present north of Ben Hai River.
This is an unlikely scenario, not even discussed above. Interesting enough is that Red China had already established de-facto relations with US, and Mao's days were numbered, I wonder what significantly would Chinese would have reacted. Also, what Brezhnev would have thought is not clear. Certainly, airpower would prevent any USSR shipments from reaching DRV.
2. South Vietnam gained all territories lost in 1975 invasion(if any), plus any territories lost in invasions between 1972-1974 up to Ben Hai River. Therefore, Cua Viet, Dong Ha, Loc Ninh and other rural areas should be firmly held by ARVN. This needs something more than US airstrikes and ARVN high morale. Even the scenario came true, with all NVA units expelled, remaining few Viet Cong hiding inside cities should be sorted out and executed by Thieu's army. This might take some time for cities and rural areas to pacify. Even RVN was pacified, an interesting scenario would have occured: Lon Nol regime fell anyway to Pol Pot regime, as Pol Pot is largely independent of PAVN assistance by the time. Thieu's Vietnam would had bordered to Khmer Rouge. What Pol Pot would have done to Thieu's Vietnam is uncertain. Would Thieu invade Kampuchea? Seemed unlikely because no direct split of ideology between comrades could have occured.
3. South Vietnam repelled attacks of 1975, retaining territories held by ARVN between 1972-1974. Therefore, PAVN still occured in some areas of RVN. This posed a weaker position of RVN. RVN still had chances to thrive around Corps III, but Corps I and II would be threatened by NVA constantly for decade until Gorbi became USSR's master and subsequently, end of cold war. PAVN would be compelled not to attack if the repel is successful and even PAVN's children army were exhausted. (By 1975, teenagers as small as 12-14 years old were conscripted)
4. South Vietnam was not defeated, but were successful to prevent PAVN from taking over Corps III and IV, including Saigon. However, PAVN might have strategic gains in many areas, including Quang Tri, Hue and Da Nang, forcing South Vietnam to truncate its territory. Although PAVN failed to achieve its final goal, the gain in 1975 would have persuade PAVN to try perhaps 3-4 years later, possibly preventing RVN's military and economy to recover and take off before end of cold war. RVN still have chances to fall before Reagan and Gorbi were in office. If Carter were in office, he might have posed negative impact on stability of RVN as he posed in South Korea and West Germany.