The choices are either a) that general elections are held which unifies South Vietnam to the North, which 99% results in a communist dictatorship, or b) South Vietnam restores the monarchy of Bao Dai and some other anti-Communist nationalist becomes Prime Minister, most likely a member of the Dai Viet political party that provided most of the leadership before and after Diem.
The only difference I see her is that the presence of Bao Dai might lead to increased stability. Relations between Buddhists and Catholics are likely to be more amenable, and there won't be the roller coastal of PMs after Diem that destroyed the ability of the South Vietnam to resist the Communists in 1964-1966 which impelled US escalation. The question is whether this would have made a difference. Knowing what Diem did, it certainly can't be worse than if he survived. Whether the long term survival is possible, I can't say, but at minimum they are unlikely to do worse than Diem did from 1955-1963, and will probably avoid any disastrous period like the post-coup period in 1964-1966.
This may dodge one bullet, but if South Vietnam is still overpowered by the North, it all depends on what kind of general the US sends to lead any large US force. Westmoreland is probably the kiss of death regardless, but even he might do better since the South Vietnamese army is likely less incapcitated without the coups.
I think the odds are still against South Vietnam, but there is an outside chance it might survive.