The problem here is that Ukraine sees no money in being a buffer state, while considering what started the Maidan protests, Russia has an idea on what constitutes a buffer state which in no way resembles a neutral state - its more along the lines of "we de facto control it through at least some branches of government, but we choose to not make that fact official".
For one if Russia's buffer states are going to act like Belarus, that's even worse than having a direct border with Russia (and a handful of countries have borders with both), because apparently such countries can act as proxies and do things so dodgy that Russia itself wouldn't like to have it on its own reputation.
Secondly, for over a decade Ukraine was a quiet and obedient Russian "buffer state", and all it got from its tolerance of a pro-Russian ruling class was being, in economic and living standard terms, frozen in time, at a rather poor level at that, while other poor post-communist countries, like Romania, or even pro-Russian Belarus, multiplied their GDP per capita.
Russia didn't mind that its satraps are incompetent, as long as they were loyal incompetents.
So then Ukrainians got the idea that they don't have much to lose from trusting the west instead, even if they install their own satraps, there's a chance those will be more competent, not to mention the west has more money to throw around, and all the other post-communist countries that sided with the west ended up much richer than Ukraine.
Without that opening, the pro-western color revolutions probably wouldn't be as successful as they were.