Chapman's the best defensive 3rd baseman in baseball, who looks to only be improving with the bat and is coming off a 6.5 WAR season at age 25. He's on pace for another 6.5+ WAR season this year.
If you made that trade right now, you'd get 5 years of Chapman v. 7 of Vlady (I think - I'm sure you guys will correct me). 2 years means a lot - don't get me wrong, but if Vladdy's ceiling is Miggy - Miggy's only had 2 seasons with more than 6.6 WAR and averaged 4.6 WAR over his first full 7 seasons.
I think you'll get as much WAR out of Chapman's age 26-30 seasons than you will out of Vlady from age 20-26. With the contact improvements he's made, he may approach the 8 to 9 WAR per year value (recognizing he was 7 WAR over a 155 game season last year).
I'm probably underselling the age thing and whatever the likelihood of us signing and retaining Vlady for his age 27-33 years - I just think his lack of defense is caps his value, while Chapman looks like he's already better than Machado and Arenado.
Plus he's not fat as f***.