Open Drawing with Vault lisp

Hi i want to replace the _open command this works on my old PC but when i go to install on another it just run the default open

is there some variable that stops defun from redefining standard autocad commands

;	variables set
(setq tilemodevar (getvar "tilemode"))
(setq varCPROFILE (getvar "CPROFILE")) 
;	close windows
(COMMAND "EXTERNALREFERENCESCLOSE")
(COMMAND "LAYERCLOSE")
(COMMAND "PROPERTIESCLOSE")
(COMMAND "ADCCLOSE")
;	Open drawing from vault
(if (= varCPROFILE "Vault_r2014_Acad")
 (progn
   (defun c:_open	()
     (command 
       "_vltopen"
       ""
     ) ;_ end of command
   );_ end of defun
 ) ;_ end of progn
) 
;	autocad variables unset
(setvar "FILEDIA" FILEDIAvar)