As well as that, if Diem was so unpopular, where was the Popular Revolution? Why did millions not revolt en masse? Many people hated the Czar in Russia, which led to the revolution there, why not in South Vietnam?
To see how silly that argument is, if you measure popularity contests in whether they revolted against one or the other, you do realize that they didn't revolt en masse against Ho Chi Minh either, right?
Plus, if we're talking the October Revolution in Russia, even that was a lot less en masse than you seem to think.
1. The grand total of forces fighting against the Czar for the whole duration of the Russian Civil War was about 3 million. Out of which about 1.2 million died. That's it. That's all.
By comparison, the population of Russia had already passed 100 million in 1900. So that grand, en-masse revolution you imagine, actually meant only 3% of the population fighting against the Czar.
By comparison, Vietnam's total population in 1950 was 29 million people. North and South together. And while it's hard to find a total number of combatants quickly, the casualties for the North Vietname and Viet Cong included over 1.1 million dead and 600,000 missing. That is, in addition to the about two million dead civillians.
I.e., per capita, that was actually a _more_ en-masse revolt than the one in Russia. They had comparable casualties at less than a third of the total population. Depending on whether you count the missing or not, per million inhabitants between 3 and 5 more people fought against the USA and the puppet Southern Vietnam regime than Russians fought against the Czar.
So there you go. If the Russian revolution is your yardstick for popular dissent, here you have something even worse.
Even if you take just strictly the volunteers from South Vietnam, enrolment in the Viet Cong had passed the 300,000 members mark in 1962, out of a total population of 12 million. While again it seems hard to find a total number of volunteers on short notice, by the end of the war, some 250,000+ such volunteers from South Vietnam had died. (And that's not counting those assassinated by the CIA: over 40,000 between 1968 and 1971 alone. And somehow that still didn't make them love the USA

) Again if you do the maths per million inhabitants, it's about twice as en-masse as the revolt in Russia.
2. But there's an even more damning aspect in that comparison you push for. In the Russian Civil War almost as many fought _for_ the Czar's regime. Yep, about 2.4 million. And again they got 1.2 million dead. Even as unpopular as the Czar was, there were still people who volunteered to fight to preserve the status quo and/or who kept fighting to restore it long after they could have said "ah well, it's over" and went home.
Where is the comparable population fighting for South Vietnam? Where are the people volunteering to fight the Viet Cong? Any volunteers numbers I can find are at best in the tens of thousands, while the vast majority of South Vietnamese soldiers were conscripts and large numbers of them apparently very unmotivated to fight the North.
Again, it actually paints an image that it's actually bleaker than the Russian Civil War.
And, in a nutshell, that's what you get when you actually look at history as it is, as opposed to some uninformed mis-conceptions about what an "en-masse" revolt like the one in Russia looked like.