Answer:
The memorial had to blend pleasingly with its environment.
Explanation:
When we say "harmonious", it generally mean to be at peace with someone, to be together with, or together in feeling or action, in harmony.
So, when the sentence -
The Vietnam war memorial needed to be "harmonious" with its surrounding site
uses the word "harmonious" in it, it means that the memorial had to blend pleasingly with its environment. It means that it has to be in sync or in common theme or standing/ element with the others.