No, they were convicted despite the fact that a number of union radicals bribed unscrupulous individuals to pretend they had alibis.
They didn't.
Modern ballistics confirms bullets fired in the murder came from Sacco's gun, and that Vanzetti was an accessory to murder which was also a capital crime.
No legal entity that has ever considered their case on the facts or the law thought them innocent. Most postmortem examinations find fault with the process -- and should -- but don't really argue against their guilt.
We now know that those two were not hiding “pamphlets” on the night of their arrest. They were hiding their explosives. We also know that Vanzetti killed a priest in the Midwest on his way to Mexico to avoid WWI. How do I know this? From Paul Avrich’s brilliant book of oral history on Italian and German/Jewish anarchists. The Italians made Emma Goldman look like a saint! And omerta died out in the late 1980s.
We also know that the infamous bombing of Wall St. in 1919 was done by Michael Boda - in retaliation for the incarceration of S&V. He eventually escaped back to Italy where Mussolini had him killed. Nasty group!
It is rather like the rallying around Mumia abu Jamal or Leonard Peltiere