"The arguments to the vl-cmdf function can be strings, reals, integers, or points, as expected by the prompt sequence of the executed command. A null string ("") is equivalent to pressing ENTER on the keyboard. Invoking vl-cmdf with no argument is equivalent to pressing ESC and cancels most AutoCAD commands. "
So that means this will work:
(vl-cmdf "_insert" "c:/test1.dwg")
(vl-cmdf)
BTW, you do know there is a seperate forum for lisp type queries?