As long as that buffer is inside Russia, I would be fine with it as I suppose most others would.
It's not OK to suggest that countries can have a sphere of influence in 21st century. Each country chooses their alignment and alliance independently. Russia has repeatedly shown to be a poor neighbour, so it shouldn't be a surprise to them if independent countries decide to turn away from Russia.
US and Russia simply can not agree between themselves what should be Ukraine's place in the world. That kind of world order does not exist any more.