The problem was not in the creation of the rule but during the populating of the data. The values written inside the Vendor column, of the Vendors sheet, were being written by another method with whitespace, so adding a TRIM where I valued the column no longer invalidated the rule and it works.

ws.Cells[c.Value + startRow].Value = string.IsNullOrEmpty(cellValue) ? (object)null : cellValue.Trim();