as for possible outcomes now, and this is total speculation, i can see a few things happening.
vieira may have the opportunity to change his plea to guilty if things aren't going great for the prosecution and a deal is negotiated. this would result in a deal with reduced sentence, probably coupled with not having to serve the full sentence that gets imposed. he can also change his plea at any time and hope for the best. if it goes to see a guilty verdict, the DA will push for the strongest sentence.
he may be found guilty on none, one, two, or all three charges. i'm not sure what the law is surrounding this, but the option may present to serve the sentences concurrently or consecutively. he would only serve the max 12 years if he was found guilty of all three charges, receives the max sentence for all of them, and they are set to be served consecutively (one after the other). that would be the worst case scenario for him if my understanding of the charges is correct. if he has any prior criminality it will come into play. so will his initial denial of involvement if he gave a mirandized statement.
we do not know if he's elected judge or jury. in the case of a hung jury, the case would need to be tried again. if found innocent, he would not be able to be retried unless there was new evidence. i don't see this as the type of case where new evidence will come to light down the road.
from the video footage we've seen, i think one unidentified guy knocks him down the first time. the rest of the blows look to have come from vieira (i am thinking he is the guy with the white shirt showing under his sweater). kind of looks like a two way fight at first, and perhaps that is why the first guy wasn't charged.
i've seen a few comments questioning the usefulness of discussing and speculating on cases like this. but i think it's important for the system to be transparent, and healthy for society to discuss and debate the mechanics of the justice system. this is a sad situation for everybody, but still an interesting case.