“Now when all the people were baptized, it came to pass, that Jesus also being baptized, and praying, the heaven was opened,”
When all the people were being baptized, Jesus was baptized too. And as he was praying, heaven was opened — Luke places the baptism after John's imprisonment and adds that Jesus was praying when the vision occurred. As he was praying is Luke's characteristic note at significant moments — Jesus' major experiences in Luke are consistently accompanied by prayer.