They will never invade, because they could kiss goodbye to assimilating Ukranians back to Russian nation. Ukrainians are Russians (genetically, etnicly or however you want to call it), that live in the Ukraine area. Ukraine is exactlly the same as Serbian Krajina. (U)kraina stands for something like "at the edge of the border", so basically a borderland. It was literary a borderland in the west of Russia. Ukraine/Krajina is a term for military border. It was like a buffer zone, highly militarised, to serve as protection from the invadors.
As you know, nations are artificial. For example the American nation. It is artificial, because it is not a seperate nationality (etnicity). They can be Russian, German, Egyptian,... But they are all Americans. The same way are Ukrainians. They have their Ukrainian nation, but their nationality (etnicity) is Russian.
The problem is that they developed their nation (in the recent history) based on hatered of Russians. Russia of course wants them back, because they are Russian people. That is why they will never invade.Because if they do then Ukrainians will forever hate them and will never go back to their historical etnicity.
I know this very well, because we have the same problems. Other nations where created by Tito from the Serbs, to decrease the number of those who call them Serbs in Yugoslavia. From the Serbs who converted to Islam (to get privilages) during the Ottoman occupation, Bosnian nation was created. Montenegrians had their Mntenegro nation before, but they where pround of their Serbian etnicity. Tito later turned them into pure Montenegrians, and made them denounce their Serbian etnicity (well, 2/3 of them).