In 吃完晚饭后，我们去了维多利亚海港看烟花, why is 了 after 去 and not after 看?
It's an exception.
With verbs in series, when the object of the first verb is a proper noun, the aspectual 了 is placed after the first verb.
So the construction becomes: &quot;Subj. + V1 + ASP 了 + proper noun + V2&quot;
In your example, you indeed have a proper noun after the first verb:

吃完晚饭后，我们去了维多利亚海港 (Victoria Harbor) 看烟花

Another example:

我去了北京找我的朋友 —— (北京 proper noun: V1 + ASP 了 + V2)
我去图书馆借了一本书 —— (图书馆 common noun: V1 + Obj. + V2 + ASP 了）