Sharing borders with China, they have culturally-held reservations /anger /fear /disgruntledness about China's influence and effects reaching within their lands, and believes China had invaded them or ruled over them unfairly at various points from ancient times to the Vietnamese Communist era, etc etc. All the typical love-and-hate between lands that were side by side for thousands of years with endless cycles of good times and bad. Relationship status: it's complicated.

So Vietnam geopolitically aims to find other great powers—USA in this case, USSR in the past, etc—to counter balance China. Simultaneously, they have been part of the sinosphere for centuries and does know the need and the how of working WITH China too.