Or -- to put it positively, South Vietnam's problem OTL was not that the Soviets and/or Chinese were assisting Hanoi. The Americans could always out-aid the Eastern bloc. We did in fact ship in quite a lot of aid, in the form of weapons, of goods, and money. If that had been enough to balance the scales, South Vietnam would never have fallen, because spending money on the Saigon regime would never have become fatally controversial in Washington. It was never the issue OTL.
What was a problem OTL was that feeding Saigon money and goods was never enough to do even the basic job of keeping South Vietnam standing up on its own, let alone any ambitions any Americans may have had of toppling Ho Chi Minh. It was always necessary to send in American (and allied, Australian and New Zealander (I think?) and South Korean) men. On that score especially, the argument against Ho based on "foreign influence!!!" not only balanced out but recoiled absurdly against Saigon, because while the Russians and Chinese were also remarkably open-handed, considering their own domestic shortcomings, with weapons and other goods too, they never got around, or were asked, to send in legions of Russian or Chinese soldiers. And more to the point, Americans fighting and dying in Vietnam would have been dangerously controversial enough if at least they were clearly winning, but when victory was in doubt and indeed few Americans could really articulate what "victory" would look like (except for the ones who thought of it in terms of the North being reduced to a smoking, lifeless and possibly radioactive ruin) then the controversy became deadly.
What South Vietnam needs is to keep the need for American help limited to money, goods, and advisors who are nothing more than advisors. To be cynical, it's even quite all right if the "advisors" are the actual power behind the government. As long as they can recruit and motivate Vietnamese to stand against the North's Communist model on their own behalf!
It was the failure to accomplish that last on a sufficient scale that doomed South Vietnam OTL, and the Doctor's job is to change that. To do it, he needs to make it clear to sufficient numbers of ordinary Vietnamese that they will do better following his vision, enough so to overcome both the harm the VM can do them in retaliation and to overshadow any positive, credible incentives the VM and National Liberation Front might hold out for them.
So the focus should be on what the Saigon government stands for, and that they are against Ho Chi Minh because he's bad, not just because he has foreign friends. In those terms, it's quite OK that Saigon has foreign friends too, because they are helping the good side--and that makes it OK that at least some Vietnamese will benefit from the foreign largesse too which makes the whole package that much more palatable.
The question is, can the Doctor do this? What would it look like?
And his falling back on the old "foreign subversive influence" card at this point is ominous because it implies that no more than Diem, he's got nothing else at this point. He'd better get something else, or he's going down the way Diem did--or conceivably, down the way the last Saigon government did, in 1975, that is, his virtues might tragically amount to no more than that he gets to run the South throughout its history instead of the OTL revolving door of "bad puppets."
Frankly for Vietnam's sake, there are two things to hope--one is that the Viet Minh won as quickly and thoroughly and early as possible so the nation and indeed subcontinent is not wracked by the terrible wars we know of OTL, and probably the VM regime is more conciliatory and less violent. The other is, a really decent SVN government. I am obviously a skeptic about the possibility of any such latter thing, but I follow this timeline to see how it might work.
And so far it's kind of marginal. Since we've preempted the other way out of the catastrophic war for Vietnam as a whole, I have to hope things pick up for the Southern government and it soon has better arguments to make.