Since you neglected to use any commas in the second part of your question, it is very ambiguous whether you mean the part of Christianity that is Catholic or mean it to be separate from Christianity, but Catholicism is a part of Christianity, and while you do not have to be Catholic to be Christian, you have to be Christian to be Catholic. In a similar manner, you do not have to be a man to be human, but you have to be human to be a man. But either way, if find it interesting your separating them. Care to shed a bit of light on why?
I had to look to make sure I was correct in my belief after reading the answer that claimed Theravada Buddhism was the most prominent. I did not think it was, and according to my source it was not and is not. It is by far Mahayana Buddhism with Theravada being a significant religious minority along with Protestant, Catholic, Muslim, Hindu, the Bahá̕í Faith and the native Hoa Hao and Cao Dai religions. Hoa Hao and Cao Dai I am unfamiliar with, and I may have to look into them. I have studied a major percentage of the world’s religions and it is unusual for me to find ones I know nothing about, but it may simply have been so long ago I have forgotten. I am a member of the Bahá̕í Faith and that one was introduced into Vietnam in the early mid-1950's, and in March of 2007 it was officially recognized by the Vietnamese government for the first time since Communism took over.
Since I have nothing other than anecdotal evidence perhaps I shouldn’t make this a statement of fact, but there is someone I know who was in the military and told me that a major part of the Montagnards are Bahá̕ís. If you are not familiar with them, they are a very peaceful people native to the central highlands of Vietnam and were very helpful to the US military, especially the US Special Forces, during the Vietnam war, saving tens of thousands of American lives. In a meeting of Bahá̕ís several years ago I also heard something else that impressed me. I now wish I had thought to get his name, I have not seen him since, and it has been so many years ago I am not certain if it was the actual person who experienced it, but the story goes there was a young American soldier in Vietnam, experiencing his very first time in an actual battle and scared to death. In an attempt to gain some courage, he yelled the Bahá̕í greeting and was surprised to hear it returned from the other side of a clearing. But after a bit of hesitation, he went in that direction, saw a Viet Cong soldier who again repeated that greeting. They met, hugged each other and each took off in the opposite directions. While I was not there to see it, I can well believe it actually happened, and probably more than once, and the one I heard it from told it with conviction and with tears in his eyes.
For an online version of a now out of print magazine giving an excellent overview of the Baha'i Faith, (except the statistics are out of date). A newer and different printed version is available. Contact your local Baha'i community for a copy.
The next link is a statement on world peace
The next one is an example of persecution.
The next four are not official Baha'i sites or Baha'i at all, but Baha'i Ocean is a multi-religion (nine altogether) study aid I highly recommend. The two below that you may find useful, though I find some minor flaws and shortcomings in them. The last one is a pretty good free Bible study aid on its own, and has perhaps fifty different versions of the Bible and scripture. I recommend that one as well.