A vector3d type has x, y, and z attributes.
To create a vector3d value, specify it as a "x,y,z" string:
Rotation { angle: 60; axis: "0,1,0" }
or with the Qt.vector3d() function:
Rotation { angle: 60; axis: Qt.vector3d(0, 1, 0) }
or as separate x, y, and z components:
Rotation { angle: 60; axis.x: 0; axis.y: 1; axis.z: 0 }
See also QML Basic Types.