Putin will not take all of Ukraine - not necessary to achieve the borders of the old Kievan-Rus’ empire.
Best solution is to split Ukraine up. The Western part, traditionally was a part of Poland or Austria-Hungary, so is more oriented to Western and Central Europe, and I would allow them to join NATO.
The Bulk of the Ukraine, based in Kiev, would be a neutral state, that would be non-aligned. While the Eastern, more Russian parts would be free to vote on a plebiscite to either join Russia, or ally with Russia. Basically have the borders that should have been after WWI.