The Taiwanese island is the only one that has fresh water source. It also has an air strip. In another word, Taiwan has an unsinkable AC in that area. All other VN control shoals are merely rocks that won't support people and could even be under water in high tides.
Also, Vietnam has a "navy" that is more like shore patrol force. Not a real navy. Vietnam simply has no chance to win a modern naval combat in that part of SCS. It should just withdraw and worry about economic development. If its GDP per capita is 1/4 of Taiwan's, than it can think about building a modern navy that is required even be considered a player in SCS.