Vietnam and Malaysia should also be taken into account. Both of them are also active participants in the maritime dispute in the SCS as well, to begin with.When you have Indonesia and Thailand you have ASEAN, the rest is an outlier.
Malaysia is strategically located at the mouth and bordering the Malacca Strait, of which West Malaysia (alongside Sumatra, Bataan and Singapore) guards the strait's entrance into the Indian Ocean. Securing Putrajaya's cooperation to remain neutral in case of war means that China could be granted a freer hand commit some forces in the Malacca Strait and Andaman Sea regions, which is highly useful towards dealing with the highly-probable unwelcomed presence of the Indian Navy in the Andaman Islands and the aforementioned regions.
(This case is also applicable for Singapore and Indonesia.)
Vietnam isn't just bordering China's Guangxi and Yunnan, but also borders a huge portion of China's Nine Dash Line claim in the SCS with its coastline, alongside Beijing's firm allies in Indochina, i.e. Cambodia and Laos. Securing Hanoi's cooperation to remain neutral could bring not just less interference on the PLAAF, PLAN and CCG activities in the SCS by the VPN and VCG in case of war, alongside preventing Vietnam from becoming a staging ground for Western-funded separatists, extremists and terrorists to wreck havoc in the Indochina region, which may cause spillovers into Guangxi and Yunnan.
Note:
SCS = South China Sea
PLAAF = People's Liberation Army Air Force
PLAN = People's Liberation Navy
China = China Coast Guard
VPN = Vietnam People's Navy
VCG = Vietnam Coast Guard
Putrajaya = Capital of Malaysia
Hanoi = Capital of Vietnam
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