Porto were on course to take the title race to the final weekend when Colombia forward Jackson Martinez gave them a first-half lead, his 21st goal of the season.
But their chances evaporated when Tiago Caeiro snatched an 85th minute equaliser.
The results meant that Benfica stayed three points clear of Porto with one match each to play and cannot be caught because they have the better head-to-head record between the two sides.
Benfica won 2-0 at Porto in the first meeting and drew 0-0 in the return on their way to a record 34th league title, seven more than their old rivals.
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News of Caeiro's goal set off celebrations among the Benfica fans at Vitoria, where their own team had become bogged down in a scrappy midfield battle.
The match was held up for two minutes in stoppage time when several flares were thrown onto the pitch from fans in the Benfica contingent behind the goal.
(Writing by Brian Homewood in Berne, editing by Alan Baldwin)
