In 2008, at the Bucharest Summit, the allies agreed on a rather important sentence in the final declaration. I don't remember the French translation, so I'll tell you in English.
It was that Georgia and Ukraine “will become members of NATO.”
However, since then there have been no concrete technical steps along the path to membership. That became more complicated in 2014, when Russia took part of Ukraine and illegally annexed Crimea, because the argument from some inside NATO was that this would create an immediate article 5 situation.
What do I mean about that? Article 5 is what I call “The Three Musketeers clause” of NATO—all for one and one for all. If a NATO ally is attacked, other allies are meant to come to that nation's defence. It complicated it politically, and yet that language stayed about Ukraine becoming a member of NATO. What NATO allies have done consistently since 2008, and even more since 2014, is provide a very significant program of support for Ukrainian military security, and also, outside of NATO, there has been a lot of support on the civilian front.
What will happen in Vilnius? As I said, I don't expect an answer to that question about Ukrainian membership, but I do find it really significant that Jens Stoltenberg said yesterday that Ukraine's rightful place is in the Euro-Atlantic family and that its rightful place is in NATO over time.
The debate at the Vilnius summit will be crucial. I don't expect answers at that time unless something miraculous happens with the course of the war between now and Vilnius.