vec1 = view3d_utils.region_2d_to_location_3d(context.region, rv3d, Vector((self.startx, self.starty)), (0, 0, 0))
vec2 = view3d_utils.region_2d_to_location_3d(context.region, rv3d, Vector((self.endx, self.endy)), (0, 0, 0))
vec = (vec2 - vec1)

From my personal experience, mainly modeling art statues (but this goes as well for any other object you model) one of the holes in Blender's workflow is when you want to scale some feature of your character/model in a diagonal direction (stretching it across one freely chosen axis). Blender only provides tools for scaling across the X or Y or Z axis, which works nice as long as the axis you want to scale along is among the Global/Local/View or Normal orientations. In many cases you want to scale along an arbitrary axis, which simply isn't accounted for in Blender. For instance, you have a character looking up diagonally and you want to  make his nose a bit longer.  How to go about this?  Previously I handled this by going to User Preferences, switch to Trackball view rotation, rotate the view painstakingly until the character's nose was oriented horizontally, switch to View orientation, choose scale origin by placing the 3D cursor, switch to 3D cursor pivot, then finally scale along the X axis, switch back to previous pivot mode, switch back to Local orientation and finally got to User Preferences to switch back to Turntable view rotation. A horribly complex way of working!  And for me it breaks my artistic flow by introducing the need for a complex counter-intuitive approach to a really rather simple modeling requirement. So that's why I created FastScale, it does this simple operation in one simple mouse move!

FastScale, a one mouse-drag scale operation.

This addon solves the hiatus in Blender described above, by offering a simple button that starts a scale operation along an arbitrary axis, parallel to the view.  You click on the mesh on the spot where you want your scale operation to originate (skipping the need for using the 3D cursor), drag the mouse to scale up or down along the direction you drag, release the mouse button and you're done!  Simple, intuitive and non-distracting.