Make sure you are upgraded to the most recent release of VASL and have the newest versions of all the official boards and the LOS tool works very well for the most part.
Here's how you move your LOS line up and down in elevation:
Keystroke... Moves... --------- -------- Up arrow... the target location up. Down arrow... the target location down. Ctrl+up arrow... source location up. Ctrl+down arrow... source location up
It can be an amazing thing when you are moving your LOS line up and down at both target and source when you are playing on one of those crazy maps that have multistory buildings on multi-level terrain.