Vinnie obviously has Jenova cells in him. A helluva lot, too. That's what triggers his limit break transformations. And if he just sat there "sleeping" in a coffin for all of Sephiroth's lifetime, but doesn't look like an old man, it's obvious that he was either dead, or constantly reconstructed by the jenova cells in him. If they can keep that guy unchanged for that long, they sure as hell can reconstruct Zack after he's taken several bullets damage. Remember, he hasn't completely recovered when you see him. he's still got a very similar syndrome to that which cloud had after the mako poisoning. That could be due to the intense amount of jenova activity in his body trying to repair him, which likely could have been going on within Cloud when he got "mako poisoning". As for Vincent not being called to the reunion, he WAS called. When Cloud first tells him their whole story with sephiroth, vincent just wants to go back to sleep, and says something along the lines of "more nightmares will come to me now". And according to him, he deserves the nightmares. But when they leave the basement, he just goes and joins them without gathering any information further than what they'd already told him. His motives for changing his mind weren't made clear, at least not that I remember. So it's quite possible that the Jenova cells in him subconsciously made him join them.