I don't know how stable a foundation the RVN was on exactly, but I'm willing to guess that it would take a lot of work to have it survive with the Vietnam War occurring as in OTL.
If I'm not mistaken, documents from the State Department around the mid-50s had the top State people involved claiming that if they really did allow free elections, 80-90% of the South would have voted for Ho Chi Minh to be their leader. So, not a promising start for a state set up for the express purpose of being anti-communist.
Other than that, you'd need more competent, non-corrupted or dictatorial leaders who could appeal to the majority of the South Vietnamese people(think the opposite of Diem). And for the Viet Cong and North Vietnamese to either bungle up the war efforts or a wiser American/western approach and involvement in the war that could entail continued American funding and support after the end of OTL American involvement in 1973.