The most common and most likely answer to this crossword clue is the 12 letter word COCHIN-CHINA
I've seen this crossword clue in The Times
C
O
C
H
I
N
-
C
H
I
N
A
Buddy explains!
The straight part
We know that "part of Vietnam" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of vietnam = cochin-china.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.