You can create a Vendor Part for an existing Vendor by doing the following:
here is a screenshot of CreateInput (I tried to create and set compoundCreatinput with _referencePropname='vm0vendor_ref' but that did not work either, so ignore the error, its just for displaying what i tried.)
But i am also not sure how else the vendor can be added on the creation ( i have the Vendor as Input parameter WorkspaceObject but not sure how i can Link it to the VP at creation of a Vendorpart Workspaceobject.
PS: to confirm i use then the createobject java action--> DataManagement/createRelateAndSubmitObjects2DataManagement/createRelateAndSubmitObjects2
Looks like you're trying to create a Vendor Part, which must be linked to a Vendor at creation.
If you use Teamcenter web client and show developer tools, you can see how the Vendor uid is sent as a property value during creation.