In English the most common meaning of "relation" is "a relative". A person to whom you are related as in a cousin, etc.
The word a native English speaker would use is "relationship".
Why is relationship between.... not acceptable?
"the relations between Vietnam and China are very complicated" should also be accepted.
I know right. If there's a foreign power involved in Vietnam, Vietnam and China are allies. But once the foreign power is removed Vietnam and China are enemies.
Why is it wrong? That's the status quo between these two countries as I understand it.
Oooh damn we gettin' political here. Can't wait to see what's next.
ARE should be accepted here
No, because the "relation" is singular, so "Tobe" is singular.
No, because the "relationship" is singular, so "tobe" is singular.
Relationships ... seems the correct way to say that in English, IMHO.