The most common and most likely answer to this crossword clue is the 8 letter word LIEGEMAN
I've seen this crossword clue in The Guardian
L
I
E
G
E
M
A
N
More clues leading to the result LIEGEMAN
- Person showing loyalty — one, say, to be embraced by old lover
- Claire regularly takes foreign language, not the right subject
- Loyal follower – chap from Belgium?
- Chap from Belgium is a loyal subject
- Subject of fiction with grand title elevated
- Story by European, not right for loyal servant?
- Subject of fiction with grand elevated title
- Loyal follower — bloke from Belgian city?
- Supporter‘s position backing celebrity golf
- Invent knowledge about degree subject
Buddy explains!

The straight part
We know that "Vassal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vassal = liegeman.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.
What does LIEGEMAN mean?
LIEGEMAN noun- A male liege (subject of a sovereign or lord).