The Croatians, who were prisoners of war, were separated into groups of seven to eight and sprayed with machine-gun fire before their bodies were dumped into a mass grave.
Those showing signs of life were shot in the head with pistols.
Vesko Krstajic, the chief judge, said: "The defendants are guilty because they killed, tortured and inhumanely treated the war prisoners."
Bruno Vekaric, the prosecution spokesman, said: "The seven maximum sentences should represent some satisfaction for the victims' families."
Five of the 18 defendants were acquitted in the rulings, made at the end of a retrial which was ordered after the supreme court overturned the original verdicts reached in 2005.
Prosecutors said they would appeal against the acquittals.
Some of the victims' family members were not satisfied with the verdict because of the acquittals.
Marica Skulic, whose 26-year-old son was killed, said: "This verdict won't get me back my son. My only hope is that it will help that such crimes never happen again."
Croatia's 1991 declaration of independence from the former Yugoslavia triggered a rebellion by its ethnic Serbs, who with Belgrade's backing captured a third of the republic's territory.
The rebellion was crushed in 1995, and Croatia recaptured the territory.
While most Croats captured by Serbs in Vukovar were eventually released, about 200 were taken from a hospital and gunned down at the farm in nearby Ovcara.
Two years ago, the United Nations war crimes tribunal in The Hague convicted two Serb officers, Mile Mrksic and Veselin Sljivancanin, for their part in the massacre and sentenced them to 20 and five years in prison respectively.