The Dockers argued at Tuesday night's Tribunal hearing the charge should be downgraded to high impact, which would have reduced the penalty by one game.
Vlastuin was left with a broken nose and Tribunal chairman Jeff Gleeson KC said he could have suffered a more serious injury.
"It is apparent from the video footage that the force here was very considerable indeed," Gleeson said in his verdict.
"The players met at speed and Voss swung his arm at speed.
"The extent and nature of the force of impact and the fact that full force of the impact struck Vlastuin's face leads us to find that the potential for more serious injury was real.
"The video evidence shows quite clearly that this was a forceful blow to the head, and one that experience and common sense indicate could have readily result in a concussion."
Dockers advocate Tim Hammond SC pointed to Richmond's medical report, which said Vlastuin was not concussed.
The Tigers added Vlastuin will not miss any training and is available for Saturday night's game against Gold Coast at Marvel Stadium.
But after deliberating for nearly half an hour, the Tribunal jury ruled the original penalty would stand.
Voss attempted to lay a tackle on Vlastuin during the Dockers' 61-point win on Sunday, but caught the Tigers premiership player across the face with a swinging right arm.
Vlastuin collapsed from the impact and was sent from the field under the blood rule.
The incident was graded as severe impact, high contact and careless conduct by match review officer Michael Christian.
Tigers midfielder Tim Taranto remonstrated with Voss, and Vlastuin made a swinging arm motion when he described the incident to medical staff as he was helped from the field.
He was later spotted chatting amicably with Voss after the full-time siren.
In the Tribunal hearing, AFL advocate Amara Hughes said there were six reasons why it was severe impact, including the risk of a more serious injury.
The Tigers pointed to a Tribunal case two years ago when Port Adelaide's Willie Rioli faced a three-game game for striking Essendon opponent Jordan Ridley.
While Ridley missed a game because of his injury from the incident, Rioli's charge was downgraded from severe to high and his penalty reduced to two matches.
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