It's funny that you would bring up Vietnam because there is a parallel there, just not the one you think. The regime in western Ukraine that US backing bears a lot of resemblance to the regime US was backing in south Vietnam, and it's now collapsing in exactly the same way.

What you don't seem to understand is that there was already a civil war between western and eastern Ukraine since the coup in 2014. Let's take a look at a few slides from this lecture that Mearsheimer gave back in 2015 to get a bit of background on the subject. Mearsheimer is certainly not pro Russian in any sense, and a proponent of US global hegemony. First, here's the demographic breakdown of Ukraine:

here's how the election in 2004 went:

this is the 2010 election:

As we can clearly see from the voting patterns in both elections, the country is divided exactly across the current line of conflict. Furthermore, a survey conducted in 2015 further shows that there is a sharp division between people of eastern and western Ukraine on which economic bloc they would rather belong to:

Ukraine is not some homogeneous blob, but a country that's divided precisely along the current combat lines.