in the LVS mod, by default the fire option is bound to "Enter" instead of "Mouse 1"
To change the weapon's key bindings:
1) type lvs_openmenu in the console, or press "C" to open the context menu and press the LVS Settings on the right side of the screen.
2) Go to "Controls" and scroll down until you see the "Armament" section, find the "Attack" bind and change the scheme to what ever button you'd like.