Here's one way to do it.  This assumes your table has a Class Name of myTable, the field in the table with the vendor names has Class Name of myTableField, and the drop-down field has a Class Name of myDropDown: 

$(document).ready(function() {
  
  //When the form is loaded, call the function to populate the existing table data into the dropdown
  loadTableDataToDropDown();
  
  //When the table is changed, call the function to populate the table data into the dropdown
  $('.myTable').change(loadTableDataToDropDown);
  
  //This function is called to load all the values from the table into the drop-down
  //the options in the drop-down will be cleared, and the new options from the table
  //will be loaded.
  function loadTableDataToDropDown()
  {
    $('.myDropDown select').find('option').remove();
    $('.myTableField input').each(function() {
      if ($(this).val() != '')
      {
        $('.myDropDown select').append(new Option($(this).val(), $(this).val(), false, false));
      }
    });
  }
  
});