Doug:
I am using PO 9.2A (150217) and I tried this with Sample Company.
The "IC Item Vendor Check" is set to Manually.
I changed the field to: Use selected vendor, but I do not see any difference.

We are not having an issue with changing the Vendor on the Header.
What we would like to do is take a posted Order that refers to Vendor01 and change the Header, and all the Detail lines as well, to Vendor02.
We can only do this currently by editing each line one at a time.