I wrote this for another forum a couple of years ago, I believe is still valid:
Vietnamese Spratly Islands multi layer defensive system (this analysis applies only to the bigger islands):
First line of defense (up to 150 km range): EXTRA (long range guided rockets).
Second line of defense (up to 40 km range): Accular guided rockets. 130 & 122 mm howitzers.
Third line of defense (up to 10 km range): Kornet anti tank missiles, Light tanks (PT-76) or medium tanks (T-54 / 55). The islands have a sizable forces of enclosed T-34-85 and even M-48 tanks, they're protected by hardened steel case and concrete bunkers.
Fourth line of defense (up to 3 km range): direct line of sight light artillery (23 / 37 / 85 mm), MATADOR anti tank missiles, other anti tank missiles, RPG type weapons (RPG-7, SPG-9, RPG-29), AGS-17 grenade launcher, 12.7mm heavy machine guns.
Against helicopter assault: Shoulder mounted Igla missiles, 23 / 37 mm anti aircraft guns and even anti tank and RPG weapons.
Air assets: The islands can deploy armed helicopters.
Passive defense: The island have steel-reinforced concrete obstacles around the islands, like the poles in Bach Dang river and/or steel obstacles on Normandy. That would slow the enemies vessels down, also act as artillery marks. So, the artillery still has its use, but its not really effective against fast moving landing boats.
Conclusions: Most things are small, mobile and easy to hide and protect, but the artillery / tanks in bunkers are vulnerable to enemy shelling.
Notes: Large weapon systems (anti ship missiles, air defense systems, etc) are too difficult to hide and too vulnerable against the enemy initial shelling and or missile attacks. Also, deploying sophisticated systems like anti ship missiles, etc, those systems have to be maintained, but can't do that properly in a small island. And lets not forget that the environment in those islands is very nasty to equipment, very nasty marine environment.
If you are going to attack these islands, you are first going to deploy plenty of assets and you are going to cut them off and then apply heavy suppression fire to destroy most of what is there and then you make your move. Its very difficult to defend against that, you can have landings coming from all directions and by air.
Now that the chinese have large bases in their newly expanded islands only makes the situation far worse. The already have 3 large air bases from where they can deploy a significant number of fighter jets as well as AWACS aircrafts. Its just a matter of time until they deploy large air defense systems which will cover the air space of the Vietnamese islands. They can also deploy long range rocket launchers to shell the islands, Vietnam has no defense against that, so there is no point in deploying large, sophisticated systems, they can't survive.
Look at what happened on the Falkland islands and those islands are far bigger, the Argentinians deployed 15,000 men and heavy artillery, but in the end they surrendered because they were cut off and had no chance of getting supplies. Actually, precisely because they deployed too many troops, the supply issue was even bigger. You can only deploy so much in an island that is 60,000 s/m (that's the biggest island, Spratly island, although its getting expanded now).
In my view those islands are not defensible against a big power like China. You can only try to do some damage to the attacking party before you lose the island, but you can't keep them. That would
require air and naval assets that Vietnam does not have and also long range missiles.
Still, only so much can be deployed in a very small island and in the end, can't stop a large landing force. Remember all those Japanese islands during WW2, all much bigger and with a lot of Japanese troops, but all were taken. Islands are not defensible against a superior enemy that can cut off the island and keep a blockade.
That's also why all those chinese islands can be easily destroyed / taken by the Americans if they want to.
