It needs to be evaluated at a higher level that what will ensure that Vietnam is able to protect itself. It should be something like below;
Scenario a) If China solves disputes peacefully and as per international laws. (no need of any weapons).
Scenario b) If China desires to use military to attack and take over SCS, then provide enough weapons to Vietnam that DETER China from launching such a attack. The idea is to make China desist from attempting such an act. Now if missiles like Brahmos/Agni (under new names) are enough to deter China from the attack then thats what should be provided and no need of nukes. but if the assesment is that China will not be deterred and still attack despite Bhahmos, Missiles then Nukes must also be provided.
So what weapons to be provided under scenario b) would depend upon what is the level which will deter Chinese from attacking Vietnam.