Hi Larry
This should work without any issues. I am not sure what is wrong in your case. Try this which might work.
Delete the source field assignment and once you choose the rule type, don't type anything in From Attr. of. Press F4 input help, it would automatically give you the screen like below. Choose 0VENDOR from that selections screen.
I just tried in the same 0PUR_C01 cube with country as the target field. It didn't any dump while activation.
If this doesn't work, try deleting the existing transformation and create a new one.
Let me know if this is helpful.
Regards
Karthik