Vassals were loyal to the person(s) above them. Therefore a king may have a vassal who may have other vassals loyal to him. A vassal has power over his serfs, or his vassals. Also the vassals have power over the peasents.

Vassals had to serve lords because the vassals promised to serve a lord in exchange for land.

The plural of vassal is vassals.

As in "our vassals have sent us gifts of silk".

In many cases, vassals did have their own vassals, in a process called subinfeudation. There is a link below, but it does not give as much information as I would have liked to find.

The dressing of the vassals was a tunic that had covered their leggings.