I tried to reproduce your situation by creating a data set like this:
Notice that field original_vendor is empty in rows 2 and 5 what Tableau interprets as NULL value
Under this situation your formula does not work well. If this your specific situation, I suggest you to change the formula to:
IIF( ISNULL([original_vendor]), [vendor],[original_vendor])
I hope this help.
You may try deleting the extract, then re-generating it after all calculated fields are created. This KB article provides this suggestion for a related problem: "Ignoring calculated field, field is already defined by data source" Error | Tableau Software
The "cause" listed in this article has not always been true when I've had issues like this--but regenerating the extract generally resolves any problems I've encountered with calculated fields in extracts.