Log in as Admin > Customization > Lists, Records & fields > Entity fields > New
• Label would be how you want to call the field, eg. Assigned Purchaser/Negotiator
• Type=List/record
• List/Record=Employee
• Apply to: check "Vendor"
• Display tab > subtab - this is where you want the field to show up on the vendor form/record
• Save
If you have custom forms might need to make sure the new field is showing. You can also move the field around by editing the form.