In a defensive, Vietnamese Navy + coastal defense + air support can wreck massive damage for the PLAN's South Sea Fleet. The Chinese know this, thus the reason why they would prefer to evade in a coastal conflict, but in the open seas where Vietnam's coastal defense and air support are either mitigated and or can be negated.
Vietnam's defense procurement program enables her to have an potent defense against even the PLAN. Provided they don't go beyond their ASI. This is the very reason why both sides defer conflict and settle this diplomatically.
Of course. But I hardly doubt the Vietnamese would harken to a Vietnamese Admiral who would send missile boats that far out to sea , far from their supply lines. The Vietnamese, if one reviews their military history, have always been defensive, and if ever they engage in limited offensives, their establishment takes are not to overstretch their supply lines.
Vietnam cannot afford to initiate offensives against a military power like China. Defensively, yes, thats a different matter.