I did not know that " Quan trọng" can be used for it is important. My answer began with Nò lá quan trọng..." so how do you know?
Same with me... :(
"Important is you happy"? That's the literal transaction. Is that really how it would be said in Vietnamese?
Why not "Quan trọng bạn là hạnh phúc."?
Because hạnh phúc is an adjective, and an adjective is a predicate unto itself and will never take là out in front of it.
"Important (it) is, (that) you (are) happy."
That's right : "Điều quan trọng là bạn hạnh phúc." Một thông điệp của sự quan tâm (A message of concern for each other.)