Serfs were a subset of peasants from what i understand https://en.wikipedia.org/wiki/Peasant

And the peasants who leased the land were his foot soldiers

The dependency of manoralism and vassalage is a little more complex.

Working for the lord was often a necessity if your land alone couldn't feed your family. Sharecroppers would get (as the name implies) a share of the crops they harvest. Other forms of labour might be paid in kind (food, resources) or in money (which might also be required to pay taxes).

The lord was expected to help out in times of crisis. If the harvest sucked, he would have been able to procure surplus food from elsewhere and help you out (putting you in his debt, of course). If there was danger from a belligerent neighbour, he would have been able to call on his liege to defend his holdings.

That in turn came with the expectation that, should your lord call on you to help, you would oblige. Your town might be expected to supply a few men, for instance, who would fight with that lord. That lord might in turn be answering the summons of his liege to defend some other lord's lands, or wage some other war for some other nobles wealth and glory

So the lord wouldn't fight alone, but use his own relationships to secure help for you, in exchange for your own service to him. In theory, that's a mutually beneficial arrangement. In practice, you didn't get much choice about arranging it.