The sentence seems to be missing a word or phrase after "united." It is likely that the intended sentence is something like:
* He united the threads that held them together.
In this corrected sentence, "threads" would be the word that means "vine." A "thread" can be a metaphor for a connecting element or a weak bond, similar to how a vine can be used to hold things together.
Without more context, it's difficult to say for sure what word was meant to be used in place of "vine."