Vucevic will lift the floor for the team, which is currently a play-in shoo-in with the lot as of now. If Vucevic joins the Spurs, he will add a potent offensive weapon to the starting unit while Poeltl will play spot minutes. That should be good enough to make the playoffs. Thirdly, his salaries drop in the next two years and he is 30 and at his peak, so he is no Aldridge at the late end of his career.
Now, if the Spurs do want Vucevic, they have to get the Magic to bite on a decent deal with a mixture of expirings and young talent. Short of selling anyone from the core of White, Murray, Keldon, Vassell, Samanic and Poeltl, this is the best option. Walker has underperformed relative to his potential and the Magic might think that its the case because of the guard glut with the Spurs. Given a greater reign at SG, he might do better. Besides the Spurs can spare that money to be paid to him from 2022 onwards for Keldon.
The Spurs need an all-star to go and help its young core. They can't forever hoard mid-late first round picks and expect development alone to push them to the top. Vucevic seems to be a modern big who fits the bill for the short-term. The Spurs can let go of Aldridge and try to woo free-agents in the offseason with cap space, but with most stars/ superstars already set in their respective teams with long contracts and the only ones remaining - Beal for e.g. being mostly guards, a position that is well filled within the Spurs, I think they will take Vucevic if they can get him. I doubt it though; the Magic are more likely to trade Gordon / Fournier than Vucevic, is what I feel.