I like Vucevic and Harkless, this is an interesting idea. Firstly I'm not sure Orlando takes this deal. Cousins, for all his potential, also carries a fair amount of risk at this point in his career whereas Vucevic has been a real bright spot for them and Harkless has already shown that at least his defense is translating. I think you're in the vicinity of something plausible though, it just comes down to whether you believe Vucevic is for real this year, whether you believe Cousins has topped out or still has room to grow, and how much value you put on a SF who does most of his damage on the boards and on the defensive side of the floor. I haven't been watching Orlando so I'm just looking at the stats for Vucevic, but I did see him play at USC and I think he has more offensive potential than he's shown so far in his NBA career. He's a top 5 rebounder this season but is that sustainable or is that because of the team he's playing on? I could see this deal going either way -- I don't think it's overly lopsided one way or another. And lacking a crystal ball, I'm not going to presume to know which of these guys will have the better career.
My gut says though that Cousins is worth holding onto, Orlando probably feels the same way about Vucevic, and I've love to have Harkless (he was near the top of my wish list in the last draft) but I think there might be a way to get him which won't cost us our franchise center.