I don't think we have an alliance with Vietnam. AFAIK, there is no US base or military agreement or anything indicating a higher relationship than trade partner.
In fact, a few times DC sent some diplomats to Vietnam, the govt over there responded by sending their diplomats to China. Basically saying they prefer China over the US. It makes sense to them since China is closer and have higher trade value than the US. But because of that I don't see how Vietnam can be a "strong ally" to the US.
Can you explain more or cite your sources?