In some variations, a process for converting a biomass feedstock into a pretreated biomass material comprises: providing a biomass feedstock containing cellulose, hemicellulose, and lignin; introducing the biomass feedstock and a recycled vapor stream to a biomass-heating unit, thereby generating a heated biomass stream at a first temperature, wherein the recycled vapor stream is at a first pressure of at least atmospheric pressure; feeding the heated biomass stream to a biomass digestor operated at a second temperature and a second pressure to pretreat the biomass feedstock, thereby generating a digested stream comprising a solid-liquid mixture and a digestor vapor, wherein the second temperature is higher than the first temperature, and wherein the second pressure is higher than the first pressure; recycling at least a portion of the digestor vapor to the biomass-heating unit; and recovering or further processing the solid-liquid mixture as a pretreated biomass material. Many variations are disclosed.