Vettel, who qualified eighth, was docked three grid positions and given a single penalty point on his superlicence after he blocked Alonso at the end of the Alpine driver's fastest lap in Q2.
Vettel, who drives for Aston Martin, was at the back of a queue of cars waiting to start a flying lap and ended up getting in Alonso's way as he exited the final corner (Turn 10).
The incident ruined any chance Alonso had of making the top 10 in qualifying, while Vettel made the cut for Q3 thanks to a lap he had set earlier in Q2.
After reviewing the evidence and speaking to the drivers involved, the stewards issued the following statement regarding Vettel's three-pace grid penalty.
"Alonso was on his fast lap just before the end of Q2," the statement read.
"When he approached the last two corners (9 and 10), there was still a queue of three cars preparing for their final qualifying lap.
"Vettel was the last car in this line and impeded Alonso and, as a result, Alonso had to abort his final qualifying lap."
Valtteri Bottas and Carlos Sainz, who were in two of the cars ahead of Vettel on track at the end of the lap, were both investigated for driving unnecessarily slowly between Turn 9 and Turn 10, but the stewards determined neither driver was to blame for the situation.
Speaking to media after the session, Vettel said he was unfairly shuffled to the back of the queue of drivers wanting to start a lap between Turn 8 and Turn 9, only for those cars to slow down in front of him between Turns 9 and 10.
Vettel made reference to an agreement between the drivers and FIA race director Michael Masi that those wanting to start a lap would slow down to find space between Turn 8 and Turn 9 but ensure they were at full speed between Turn 9 and Turn 10.
"I think from my side, I didn't do anything wrong," Vettel said. "I was the last one in the queue. But I think the others should not jump the queue in the place where we agreed to slow down to then slow down after that. So I don't know if they were not listening.
"We agreed that we slow down between Turn 8 and Turn 9, which is what I did, but everybody passes me and jumps the queue," Vettel added. "And then they all slow down between 9 and 10 where we agreed not to slow down. So now you know the rest of the story."
"As soon as I looked in the mirror [in Turn 10], because it's not a nice place to meet some traffic, and I saw him [Alonso] coming, like half a second later, just after I saw him and sped up, I got the radio call. So I tried to avoid him, and basically cancelled my lap."
Max Verstappen starts on pole with Lando Norris lining up on the front row for the first time. Red Bull teammate Sergio Perez starts on the second row alongside Lewis Hamilton.