Why does Vietnam want US weapons?

As US President Barack Obama announced the lifting of the decades-long embargo on sales of lethal weapons to Vietnam, he seemed at pains to explain the decision "was not based on China or any other considerations".
Tensions have ratcheted up in the region as China has reclaimed land in massive dredging operations, turning sandbars into islands equipped with airfields, ports and lighthouses.
Now Vietnam will be able to access the latest technologies and equipment the US military has to offer.
the US announcement is a powerfully symbolic gesture - "proof that the US-Vietnam relationship has fully normalised", as Vietnamese President Tran Dai Quang put it.