He wasn’t Constitutionally acquitted by the Senate because the SCOTUS Chief Justice did not preside over that farce.
“By that logic, the Senate effectively vacated the charges by holding a fake trial.”
The Senate holding a ‘fake’ trial doesn’t vacate criminal activity. The trial didn’t have a chief justice, because they were trying to remove someone from office that was already out of office. IE, it made no sense.
It actually made a lot of sense. If Trump had been convicted in the Senate, they would have sought to disqualify him from holding Federal office again as the penalty.
P.S. — Legally, the Senate did not hold a trial for an impeached President. He was an impeached ex-President, which meant it was treated no differently than an impeachment of any other Federal official (a Federal judge, for example).