You can use GRT wizard in 2 ways, i.e., knowing the vendor and another is without knowing the vendor.
If you know the vendor to which goods are to be returned, then the system restricts you from scanning if you scan item which was received from some other vendor.
In case you don't know the vendor, then the system allows to scan all the items and the system automatically creates different GRT documents.
You can use the same in order to get prompted for vendor mismatch.
We shall keep this idea open for others to vote and if it gathers enough vote, then we shall definitely consider it in our roadmap. We are marking this idea as "Unlikely to implement".