All around the world in history, a smaller nation always views the larger and more powerful nation as a threat, and this is because they are a threat.
It’s no different than Ireland seeing England as a threat, or Taiwan seeing China as a threat, or parts of eastern Europe seeing Russia as a threat, or Yemen seeing Saudi Arabia as a threat, or Nepal seeing China as a threat etc.
They see them as a threat, because they are an obvious threat.
Also Vietnam continued to see China as a threat, especially when China takes it’s ships down into the seas adjacent to Vietnam and claims those are Chinese waters, fishing is a large industry for Vietnam and losing their own waters to Chinese expansion would see Vietnam crumble economically and be even more susceptible to China.
Here is a map BTW of the region.
They next map is my own edit of that map, it’s what I would consider fair claims and ownership of waters.
I base ownership on how far the water is from your coastline, I think if the water is right next to your coastline, you should be the country that has the rights to that water, not another country.
China doesn’t agree with this however, instead China claims it basically owns all of the waters in South East Asia, the below map are China’s claims to what it owns.
As you can see China’s claims are simply ridiculous, China basically claims waters all the way up to the Vietnamese coastline, which is simply asinine.
China also claims waters basically touching Malaysia and the Philippines's long Palawan island.
There is simply no way for any countries in the world to take these claims seriously, they are insane.
As you can see China also colours in Taiwan as it’s own land there, as well as Japan’s southern Islands Ishigaki, Miyakojima, Tarama, Taketomi, Yonaguni.
These islands have never been part of China in history, and they are literally part of the Okinawan prefecture. These islands are home to 110,000 Japanese people, below is Ishigaki city in Ishigaki.
As well as Miyakojima city in Miyakojima below.
China is actually stupid to claim these as it’s own, because it makes China look insane and unreasonable.
If China was a more intelligent colonist, it wouldn’t reveal it’s hand so early, it would instead just claim Taiwan first, then if it could succeed in taking Taiwan it would then claim Ishigaki and Miyakojima.
However by claiming all of these from the start, instantly China makes itself look ridiculous.
Especially claiming waters all up until the Vietnamese beach, that is also flat out insane.