First of all they are in different leagues.
Moreover there is a very strong possibility,
Vietnam may be reannexed by China as its former stolen province of Annam by the French if war breaks out between China and USA.
Any military strategist will be considering this.
How can France grant Vietnam Independence since she stole it from China?
That is why VCP maintained its close tie with China today. It does not wants to appears as a threat or a nation that is linked the conspiracy involving China's rival.
Vietnam is much aware of this dangerous chess move as instigated by Mike Pompeo.
Why should USA support a Red Communist state like Vietnam? She only wanted to use her and China will prempt USA.