While exporting I am facing the problem that in modify columns new field vendor name has been added even data is reflecting ,for the previous logged cases we are not able to get the Vendor name , what we have to do for getting the name for previous cases .
If vendor name field is blank for previously logged cases it would be because these cases were logged without populating the vendor name field. If you want them to reflect a particular value you could search for cases where vendor names are blank and do a mass update for it to save a particular value. hth
It looks like the group of read only fields in which vendor is grouped is a virtual join which shows the information from the CMDB for the Affected CI located in the ticket. Virtual Join fields are not stored in the record. Unless you've updated the fill link for the CI to actually populate the ticket vendor field when the CI is selected, it will be blank.