Can't you change it like any other file extension type?
In Windows Explorer, right-click on the vmdk file and choose Open With... Choose default program. Pick VMware Workstation, check the box to "Always use the selected program..." and hit OK.
That is the thing, i don't know which one is the default program that runs that vmdk file...
thank you for the response by the way
"VMware Workstation" (or "VMware Player" if you only have that)