Russias has its own interests vis a vis eastern Europe to look after, so Vietnam doesn't figure in that. For one Vietnam interests now align more with the US than Russia. However , that doesn't means Vietnam won't keep its ties with Russia and keep buying weapons from Russia. Russia is still Vietnam's largest military supplier by far and it will remain that way for a long time, barring a major geo political change in Vietnam. And all this despite the fact that Russia and China are now on good terms.
In fact the irony is that Russia still has an upper hand in its ties with Russi, since Russia can sell its weapons to China's enemies/adversaries like India , Vietnam etc, but Chiba usually abtains from doing so towards Russia's adversaries. I'm sure if Ukraine wanted to buy weapons from China for example, China won't agree to sell them so as not to offend Russia. But Russia can do so and they have been doing so for decades towards China.
That means you underestimate Putin, he was the former head if KGB Russia top spy agency after all. So he comes from a military and intelligence background. He has an advantage over leaders who come from a political/diplomatic/engineering background
like China, US,UK , France, Japan, Germany and powers with whom Russia has to contend with day to day. So Putin has an advantage here when it's comes to geo strategic and geopolitical games given his background advantage .
So for you to think that Putin didn't plan for this invasion for a while justifies my point. If you really think he just woke up one morning and decided to invade Ukraine, then I can only wish your leaders are more knowledgeable than you in this field.