They're permanently stunted because of the 1979 war and the resulting economic pressure after that.they only reformed 5 years after China did. They are indeed smaller, so that means they should be able to move faster. But they aren't, they're moving slower. Even a weak leader like Jiang Zemin who had a weak hand, was able to stop the PLA from participating in commercial activities in 1995. Vietnam still can't do it. Vietnamese national champions collapsed under their own debt, and they can't even ban foreign social media.
Without the return of an alliance with China, Vietnam is doomed to insolvency and irrelevance. There is simply no market that is ready to absorb their products or invest in them.
But nowadays the Vietnamese government is approaching China again. China too should make amends for damage inflicted on Vietnam when the USSR was the actual target. Perhaps not as far as giving back land, but at least a statement and monetary compensation.
Vietnam as a fellow communist country naturally belongs as a close extension of Chinese policy, just like the days when they united to give Vietnam freedom in the first place.