trigger trVendorPrefill on Vendor_Product__c (before insert, before update) {
 VendorMgmtClass.conSameAsMaster(Trigger.new); //
}

Class:

public class VendorMgmtClass {

public static void conSameAsMaster(Vendor_Product__c[] vp) {
  integer i = 0; 
  for(i=0; i < vp.size(); i++) {
   Master_Vendor__c v = [Select Primary_vendor_contact__c,
         Primary_contact_title__c,
         Primary_contact_phone__c,
         Mobile_phone__c,
         Fax__c,
         Email__c,
         Product_manager__c
         from Master_Vendor__c where Id = :vp[i].Vendor__c ];
   if(vp[i].Contact_same_as_master__c == True) {
      vp[i].Contact_Name__c = v.Primary_vendor_contact__c;
      vp[i].Title__c = v.Primary_contact_title__c;
      vp[i].Phone__c = v.Primary_contact_phone__c;
      vp[i].Email__c = v.Email__c;
      vp[i].Mobile_phone__c = v.Mobile_phone__c;
      vp[i].Fax__c = v.Fax__c;
      }  // end of if contact same as master == true
   if(vp[i].Relationship_same_as_master__c == True) {
    // More fields to copy from master record to this detail record
    vp[i].Product_Manager__c = v.Product_manager__c;
   }
  } // end of for vendor loop on trigger
 }
 
  //********************************************
  //Begin test coverage for method conSameAsMaster
  public static testMethod void testconSameAsMasterTest()
  {
       
   //Create a Master Vendor rec
   Master_Vendor__c mvc1 = new Master_Vendor__c(Name='testrec##',
   primary_contact_phone__c = '4141111111',
   email__c = 'mytest@mv.com',
   mobile_phone__c = '4141112222',
   fax__c = '4141113333');
   insert mvc1;
   vendor_contact__c vc = new vendor_contact__c(name='name##',
   master_vendor__c = mvc1.Id);
   insert vc;
   mvc1.primary_contact__c = vc.Id;
   update mvc1;

      Vendor_Product__c myVP = new Vendor_Product__c(Name = 'testvendor##',
      vendor__c = mvc1.Id,
      relationship_same_as_master__c = True,
      contact_same_as_master__c = True);
      insert myVP;
  }

  //End test coverage for method conSameAsMaster  
  //******************************************* 
}