Plausibility Check: Republic of Vietnam south of the Mekong?

Alright so I was browsing wikipedia and saw something that was interesting enough to get me to stop lurking and actually make a post.

https://en.wikipedia.org/wiki/Cao_Văn_Viên#Role_during_government.27s_final_days
At an assembly of top generals on March 11, 1975, President Thiệu declared he would abandon the Central Highlands—trading land in order to achieve a more defensible concentration of population and troops around Saigon and the Mekong Delta.[27][148][149][150] Although he had believed consolidation was necessary for some time, Viên had never voiced his concern. He finally did so at this meeting.[115][86] Yet, Viên also privately believed that the war was unwinnable if Central Highlands were abandoned.[115] Accounts of this meeting differ, however. Some versions have Viên remaining silent regarding Thieu's consolidation decision.[43][151] Whichever version is correct, the government did not prepare the army, its allies, or the public for the decision, nor did it anticipate how the decision might affect the war effort. Although Gen. Viên met with Maj. Gen. Homer D. Smith shortly after the March 11 meeting, he did not inform him about President Thiệu's decision—leaving the Americans unprepared for what followed.[2][43] Thiệu's decision led to widespread panic among the public, and the collapse of the Army of the Republic of Vietnam.[2][126][148][149][152] As panic set in and ARVN troops refused to fight or deserted in large numbers,

Could a front line on the Mekong Delta have been maintained had the withdrawal to the south been better planned and executed? If so, how viable of a rump state would the new South Vietnam be? (I'm really not familiar with the geography, demography, or infrastructure of the region) Could the ARVN hold out long enough to get a Korea style armistice?
 
Is this the equivalent of Cochinchina? I suppose the frontier would be shorter, but does that translate into greater defensibility?
again I'm really not too sure about the geography of the area but I think the area in question would roughly translate into Cochinchina (though looking on a map it seems that Saigon may be a bit north of the river delta).

In terms of defensibility the Mekong is a major river and would be a pretty good defensive boundary against a conventional enemy, so if nothing else it would put the PAVN's T-55s out of the picture.
 
About 80% of the RVN population is in III and IV Corps Tactical Zones; when the Central Highlands were abandoned, II Corps was unable to actually break contact, and was thus routed, leaving some of the army's best units in I Corps surrounded. Giving up half your army over 20% of the country's population for free is not good strategy. The ARVN gave good account of itself at the Battle of Xuan Loc, after the destruction of the right wing, so if those forces could be preserved, they could probably fight the NVA to a standstill for a much longer time.

Continued survival could make US abstention from the conflict much more awkward; getting advisors and airpower committed would go a long way towards blunting the NVA offensive, and if the US was really serious about protecting its ally, it could deploy a force to amphibiously envelop the NVA, or even threaten Hanoi [this being post Nixon in China, with the Soviets and the Chinese having previously encouraged the DRVN to back down]. Short of renewed US assistance, though, even a shortened line would only delay the fall of Saigon, unless maybe the RF/PF were trained beforehand to fight as insurgents in concert with the ARVN regulars.
 
when the Central Highlands were abandoned, II Corps was unable to actually break contact, and was thus routed, leaving some of the army's best units in I Corps surrounded.
Could this aspect of the withdrawal have been avoided in anyway with better planning or timing?

unless maybe the RF/PF were trained beforehand to fight as insurgents in concert with the ARVN regulars.
I was thinking that maybe while retreating the ARVN could leave supplies that they can't bring with them in designated caches that would be left to FULRO, though that would probably assume far more breathing room than the ARVN could have counted on.
 
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