In vietnamese, when expressing feeling using adjectives, we often add "lắm" or rất"
ex: tôi rất vui, tôi vui lắm equivalent to I am very pleased
tôi rất hạnh phúc, tôi hạnh phúc lắm = i am very happy
Tôi mừng lắm, tôi rất mừng = i am very glad
If you say " tôi vui or tôi mừng or tôi hạnh phúc" it sounds not so natural. However sometimes in a certain context, those expressions is perfectly ok for ex: when someone asks you "bạn có vui/ hạnh phúc không?" and you're so deeply happy and a bit moved, surprised, awkward, shy, etc, you might say them without adding "lắm" or "rất".
But normally, just say: tôi vui lắm, tôi hạnh phúc lắm, tôi buồn lắm, tôi mừng lắm, tôi giận lắm, etc
hope it helps.