Terrorism is now happening in many countries around the world, especially those with multi-party regimes. Just as you say terrorism causes injury to people. A series of bloody gunshots. Vietnam is the least terrorist country because it is the only Communist Party of Vietnam.
In that case, there should be no incidents of terrorism in Syria. The only legal and recognized political party there is the Syrian Ba'ath party. But it is not true. Half of the country is under the control of the ISIS, and a large section is also under the control of Al Nusra.
I think in Vietnam's case, the ethnic homogeneity did help. The Communist party already won and there is nothing else to squabble about. That's why countries like Japan and South Korea have no separatist groups. There's very little cultural and religious differences. (
With the Philippines, Muslims are also a different ethnic group. Save for inter-ethnic marriages, being Muslim is seen as an identity. Interesting to note that before the republic was created, there are areas that are now provinces that has declared independence separately from other regions. You don't see those provinces going in revolt. Why? They are Christian-majority.