Dude, stories always are both side and yes, China was big helped Vietnam during Vietnam War with US. Beside the help Vietnam, that also help China too.
US was supported Taiwan and China affraid that could tear China apart like its old day, so whatever it takes, China support all the way from Korea war and Vietnam just like to protect itself.
More than that, China took chance when North and South fighting, China Navy occupied Paracel Islands until now.
Battle of the Paracel Islands - Wikipedia, the free encyclopedia
See how nice China HELPED is?
Don't blame Vietnam turned its back to China. If you not nice to us, we will have plans to deal with.That's life.