Then you’re thinking about Vietnam backwards. What you seem to be missing here is that the civil war in Ukraine started after the western backed coup in 2014. And the regions that wanted to separate from Ukraine are where the line of contact is.
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:
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:
The regime that US backs in western Ukraine is entirely reliant on western aid both economically and militarily. This is precisely the same situation for US as Vietnam where they backed the regime in the south. Eventually, US found the cost of the war to be too high to bear and pulled out, at which point the south regime collapsed. We’re now approaching this point in the war in Ukraine. As Czech president recently pointed out, Ukraine has around six month left. After that point, western support is likely to dry up.