China only wants to maintain its sovereignty.
As for the oil/gas fields, we know that Vietnam desperately needs these resources to improve its living standard.
As you mentioned above, 50-50 or even 40-60, we are willing to negotiate, but it was Vietnam who keeps insisting that everything should belong to her.
The most profitable way is to accept China's negotiation, but those Vietnamese's unreasonable hatred towards China has basically clouded their reasonable judgement.
It is hard to convince those paranoid folks.