You’re completely forgetting the most important part…
China has already shown that they’re willing to negotiate (e.g. the Gulf of Tonkin with Vietnam, which was favourable to the Vietnamese).
With regards to the Paracels, Vietnam holds claims solely as leverage in the Spratlys. Vietnam lacks any control over the Paracels and has not supported American FONOPS through the Paracels for that reason alone. Vietnam knows that their claim to the Spratlys is strong. Vietnam has been escalating their island-building in the Spratlys for that exact reason. From what I can tell, Vietnam is trying to secure partial or total jurisdiction over the Spratlys in exchange for yielding the Paracels. Unfortunately, until Vietnam/China obtain exclusive co-sovereignty in the region, such an agreement is impossible.