Again, I think that's just down to raw ability.Also keep in mind that Vietnam didn't put up any sea walls to protect their reclamation efforts. If one looks at the picture sof China's islands, you can see sea walls there.
It's not just about building a sea wall, but you also need good foundations and sunken coastal wave breakers and other defences.
Basically, once you start to build a sea wall, it will be very costly (money and time) and technically challenging to expand beyond that boundary.
Vietnam either wasn't able build the defences in time, or they took a gamble hoping they can get away with no defences during a storm season so they can crime two years' worth of reclamation in before settling on the borders.
Again, that boils down to the speed of reclaimation.
They simply lacked the technical expertise and scale of resources that China has.