Let's say you agree that Harper was wrong to try to re-cast Canada's Vietnamese community. You would probably also have to agree that he was wrong to re-frame the Vietnam war, too, because that necessarily has an impact on the community. Canada maintained a non-partisan stance throughout all the events in SE Asia and in most eyes there is no legitimate reason to re-write that history.
If you don't agree that Harper was wrong, then there is nothing to fix. Canada's Vietnamese community will remain marginalized because they were not on the right side of something that concluded as long as 10-20 years before they came to Canada
When we speak of fixing things, we are talking about the behaviour of the Government of Canada and the politicians who represent the government.
As individuals, people are free to take any position they want on the Vietnamese war and the Vietnamese community in Canada, short of acts of outright hatred.
The government of Canada, on the other hand, has a responsibility to act rationally and fairly toward its citizens, as well as avoiding acts of outright hatred.
Since Canada was a non-partisan observer to the Vietnam war, Canada has no moral obligations to any party to the war. Canada's moral obligation lies in recognizing and accepting the diversity of political viewpoints and the diversity of Canada's Vietnamese community. That still includes the "victims of the fall of Saigon", but it would no longer exclude others.
When Canada officially makes any partisan reference to the Vietnam war it is violating its moral obligation to remain non-partisan so as not to marginalize the people that Canada previously worked with on a non-partisan basis. Among others, this includes the thousands of Canadians who assisted the boat people.
If some individuals want to mark the war in a partisan manner, that is their right to do so. But it is not the proper business of the Government of Canada to join them officially.
Canada can act on may levels to fix the problem that Harper set up.
At the bare minimum, Canada can stop referring to Canada's Vietnamese in partisan terms of the Vietnam war.
At the next level, Canada can go around and erase all such references that have become embedded on government web sites and government property, and stop funding people who are trying to embed more of them.
Ultimately, Canada could renounce all the mistakes made in recent years and Canada could repeal bill S-219, which embeds Harper's vision of Canada and Vietnam into law.
For further discussion of change, see Excuses politicians make: and how not to overcome them.