Vice President J.D. Vance said Sunday that Russia has made "significant concessions" to President Donald Trump during negotiations to end the war between Russia and Ukraine, which has gone on for three-and-a-half years. 

"I think the Russians have made significant concessions to President Trump for the first time in three and a half years of this conflict," Vance told NBC's "Meet the Press" when asked by host Kristin Welker about whether Putin was stringing Trump along.

He gave some details about how the negotiations are going and addressed the fact that Russia's foreign minister said there is currently "no agenda" for a meeting between the two nations' leaders.

"They’ve actually been willing to be flexible on some of their core demands. They’ve talked about what would be necessary to end the war. Of course, they haven’t been completely there yet, or the war would be over. But we’re engaging in this diplomatic process in good faith," Vance said.

Searching for "middle ground"

"We are trying to negotiate as much as we can with both the Russians and the Ukrainians to find a middle ground to stop the killing," Vance continued. "I think what the president has tried to do here is try to engage in very aggressive, very energetic diplomacy because this war is not in anyone’s interest."

Vance seemed satisfied with the process so far and comfortable with the fact that it would take time and might have some ups and downs along the way.

"If you look historically, whenever you have a complicated war with a lot of death and destruction, it kind of goes in fits and starts," he said with optimism. "There are hills and valleys to the negotiation. We sometimes feel like we’ve made great progress with the Russians, and sometimes, as the president has said, he’s been very frustrated with the Russians. And we’re going to keep on doing what we have to do to bring this thing to a close."

"I don’t think it’s going to happen overnight. I think that we’re going to continue to make progress," he added.

"Territorial integrity" for Ukraine

Vance didn't give a lot of details about what that middle ground might look like, but he did say that Russia has acknowledged that when the war ends, Ukraine will "have territorial integrity" as they did before Russia invaded. "They’ve recognized that they’re not going to be able to install a puppet regime in Kyiv," he said.

At the outset, Russia had demanded to keep parts of Ukraine that it was able to conquer during the conflict, while Ukraine demanded the return of Crimea to its control. Russia took over Crimea during a previous conflict with Ukraine in 2014, so it would be a step backward for Russia to give it back now.

Coming from a position of superior strength, Russia is not likely to stop fighting unless it gets something out of a settlement with Ukraine.

For its part, Ukraine does not want to give up anything to Russia that it did not have before, and knows that it can probably get aid from various countries to keep the war going for a long time yet.

Vance said, "Ultimately, whether the killing stops, that determination is going to belong to whether the Russians and Ukrainians can actually find some middle ground here."