Play with this:
(setq dbxApp
(if (< (atoi (setq oVer (substr (getvar "acadver") 1 2))) 16)
(vla-GetInterfaceObject (vlax-get-acad-object) "ObjectDBX.AxDbDocument")
(vla-GetInterfaceObject (vlax-get-acad-object) (strcat "ObjectDBX.AxDbDocument." oVer))
)
)