Defensively, Cabrera at third will not work. The last two seasons he played signifcant time at third, 2006 and 2007 when he started there for the Marlins, he committed a combined 40 erros in two seasons and had a combined UZR/150 of -7.8, which is not good at all. Then when he played in 2008 for the Tigers, he committed 5 errors in 14 games, which would have been a UZR/50 of -36.8. Obviously that wouldn't have held up because that was such a small sample size, but still, not good.
Now you're asking him to move back to third 4 years later after playing first, where he was a below average fielder, and he's a bigger guy now. That doesn't look to end well. The guy who played third the most for the Tigers last year, Inge had a 3.3 UZR/150. So it's a big drop defensively. Obviously the thought is Cabrera's offense will make up for that and more, which it should, so short term it shouldn't matter.
2 other points. First is there will also be a drop defensively at first from Cabrera to Fielder. 2 is the idea of playing V-Mart at third is insanely unrealisitc. They don't have UZR for catchers, but he was never great defensively there, is a below average 1st baseman defensively, and is older and coming off major knee surgery. Plus he's never played there in his MLB career. Won't happen.