What is a good word between 'slave' and 'servant'?

Sorry, but there really isn't a good general purpose word for this in English. There are some different words you can use for specific situations such as 'indentured servant', or 'serf', but it's a hard void to fill.

One solution used in some bible translations from older English usage is 'bondservant'.

A serf has a status in between that of a slave and that of a servant.

• A serf cannot be sold but he is bound to a plot of land owned by his lord. Serfdom was a common condition in feudal Europe during the Middle ages until the plague epidemics and abolished in Russia by the liberal tsar Alexander II in the 19th century.
  A serf is not paid, instead he is required to work on his master's land and this takes precedence over his own plot.
• A servant, in contrast, has wages and can theoretically choose his master.
• A slave—there are many variations depending on the place and times—is bought and owned for life. His emancipation depends entirely on his master's goodwill.

vassal (noun)

1.(in the feudal system) a person granted the use of land, in return for rendering homage, fealty, and usually military service or its equivalent to a lord or other superior; feudal tenant.

2.a person holding some similar relation to a superior; a subject, subordinate, follower, or retainer.

3.a servant or slave.

vassal
c.1300 (implied in vassalage ) "tenant who pledges fealty to a lord," from O.Fr. vassal, from M.L. vassallus "manservant, domestic, retainer," from vassus "servant," from O.Celt. *wasso- "young man, squire" (cf. Welsh gwas "youth, servant," Bret. goaz "servant, vassal, man," Ir. foss "servant"). The adj. is recorded from 1593.

Two somewhat regional terms that might qualify here are peon and sharecropper.

On peonage:

The origin of this form of involuntary servitude goes back to the Spanish conquest of Mexico when conquistadors forced poor Natives to work for Spanish planters and mine operators.

^Wikipedia

On sharecropping:

Sharecropping became widespread as a response to economic upheaval caused by the emancipation of slaves and disenfranchisement of poor whites in the agricultural South during Reconstruction. Plantations had first relied on slaves for cheap labor. Prior to emancipation, sharecropping was limited to poor landless whites, usually working marginal lands for absentee landlords. Following emancipation, sharecropping came to be an economic arrangement that largely maintained the status quo between black and white through legal means.

^Wikipedia