In his 4 starts he has thrown his 92.4mph fastball 67.4%, 85.2mph slider 18.2%, his 85.4 change 7.6% and hi 81.7mph curve 6.9%. This shows that he was been mixing his pitches effectively in his starts thus far. He can hit the mid 90s with his fastball, but he does so only in the early innings. His GB% is only 40.3% which is not great, but in the minors it was 50%, which is a pretty good number. If he can start settling in a bit more and using that heavy fastball to his advantage he could have continued success.
His minor league numbers aren't all that eye popping.
9.2(H/9) 0.5(HR/9) 3.1(BB/9) 6.7(K/9)
However, as the op stated, this year in AAA has been better as he cut down his BB/9 and H/9 while boosting (slightly) his K/9. At 22 perhaps he is starting to figure things out.
There are some disturbing stats that point to a recession. But I mean who counted on him maintaining a sub 2.00 ERA and a sub 1.00WHIP? His BABIP is .236, while the league average is closer to .300. His LOB% is 84.8%, while the number should be closer to mid 60%. His FIP is stellar at 3.19, but it is not sub 2.00, like his current ERA.
So what I gather from this is Vin Mazzaro can be an effective pitcher so long as he gets his heavy fastball working. He will likely not blow away too many hitters, but his K/9 will probably sit in the 6-7 range, which is not bad at all. His numbers stand to regress, but his K/9 may increase as he starts to settle in a bit. He can be an effective guy, but probably more a 4/5 guy than a 2/3. Certainly a good guy to have around, but I would continue playing the matchups unless something drastic changes.