Is vassal a word in the Scrabble Dictionary

Yes, vassal is playable

...and is worth 11 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Vassal" Definition

noun

A bondman; a slave.
A person who held land from a feudal lord and received protection in return for homage and allegiance.
A subordinate or dependent.
A person under the protection of a feudal lord to whom he has vowed homage and fealty a feudal tenant.
One in a subservient or subordinate position.