More likely Ukraine has agreed to become a Vassal State.
The historical relationship between Russia and Ukraine.
This is neither likely nor possible. Poroshenko cannot speak for Western Ukraine - and policies of Right Sector created a lot of people there who see Russians as the enemy.
What is more likely, IMO, is that Putin showed Poroshenko a documentary movie that was filmed in last couple of weeks in the theater of war. And then he showed him an invitation to Hague - a criminal complaint with charge of war crimes. That's the fire that Presidents are playing with. A government has no right to kill citizens just because they have a different political opinion. (The opposite is not true.)
Russia has other ways to apply pressure. For example, it may establish a no-fly zone over the area of the conflict. Precedent-wise, this would be something that the UN will have to accept - it hurts nobody, and it prevents further destruction. Poroshenko is a businessman, not a zealot. He will approach this from the position of logic. Ukraine cannot make people of Donetsk love Ukraine by killing more of them. As the separatists aren't marching on Kiev or doing any other offensive, Ukrainian Army cannot justify its actions that would be fit for Soviet Union in 1968, not for a modern democracy that strives to join EU.
The tit-for-tat game cannot continue indefinately. There has to be some agreement or the entire Eastern Ukraine will continue to be destroyed. Which I do believe was Porsh’s. It’s the old story if I can’t win then you’re not getting it either run.
Ukraine army continues to refuse to fight so much so that Porosh has ordered ALL the police to the front under threat if they don’t go they’re fired.
He has no choice but to continue the fight or negotiate with Russia....just as you say Ukraine will again be a Vassal State.