Wow. Yes. That person has issues.

Russia sanctioned-proof themselves in anticipation of the consequences of invading Ukraine. They have long history and experience after all since the Soviet days of international sanctions. However, autarky always shows to only have short term effect but in the long run, the severe consequences always catches up in the end. Just look at how Soviet Union led to its eventual demise and the war in Afghanistan accelerated that collapse. The war in Ukraine will do the same to the current regime.

Conversely, even though the EU had been dependent on Russian fossil fuel before, the boomerang effect you mentioned is only short to medium term because disruption in trade is always expected during a war. But this only pushed the EU to import American gas and accelerate the EU Green New Deal to compensate for the loss of Russian gas and oil.

With Russia only having 1/5th the GDP of Italy and endemic corruption, the failure to subdue Ukraine is only going to eat at the Russian economy and political prestige. Even if Russia wins or gets concessions, it will be a Pyrrhic victory. Because loss of demographics (Russia is experiencing population decline before the war) will have severe consequences to the workforce and economy, the sanctions after the war will continue to hit the Russians, and the country will become more economically reliant to China and by extension politically as well. More importantly, the claim to stop NATO expansion just had the boomerang effect of just admitting two new nations into the alliance. With Russia tied in Ukraine, they loss influence in CSTO as member states resumed border clashes, especially with Armenia becoming frustrated on the issue of Nagorno-Karabakh.

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.