IF no one helps then sure they would win fairly easy because Vietnam and Philipines have extremely limited air and naval capabilities BUT that is not realistic. Any aggression by the PLAN toward another sovereign nation especially Phillipines and Vietnam would most definitely bring forth the navies and air forces of the US, Japan and South Korea to bear in which case it will go very badly for the PLAN or ....ultimately escalate to WW III which then everyone loses.
Who intervened when China invaded Vietnam in 79? Why are people so quick to assume that Uncle Sam is going to deploy its 7th fleet to safeguard a few Vietnamese and Filipino islets from Chinese "aggression"?
Japan and South Korea? Why in the world would they have even the slightest inclination of risking their enormously profitable trade relations with China over a few reefs thousands of miles away from them? If you think the Japanese economy is bad now, wait until China stops buying their stuff.
The US did not intervene when Russia beat down Georgia, which was practically a protectorate of the US.
Even if the US was inclined to intervene, how do you think Taiwan is going to feel about the US helping foreign states take over what they consider to be Chinese (PRC or ROC, doesn't matter) territory?
And what exactly does the US have to gain from intervention? That is the most pertinent question. The reason the US is stirring up troubles in the South China Sea is precisely because they are wary of a direct military confrontation against China. Do you really think the world's two most powerful nations are going to start a war over some insignificant islets?
If Vietnam and Philipines think the US is going to help them defend their claims, they're sorely mistaken. They are nothing but pawns.
All this is not to say that China is going to start a war. However, if things do escalate in the region and if China plays their cards right, we could very well have a South Ossetia moment in the South China Sea.