A better way to do conditional active record statements?

I'm trying to figure out a better way to have one query here. I want to be able to send something to last where statement a wildcard so I can select all vendors. Right now if i don't include that line it doesn't filter by vendor so I essentially get all the purchase requests.

Any thoughts of a cleaner way to do these sorts of queries?

  if @vendor == "0" #checks for vendor
    @purchase_requests = PurchaseRequest.includes(:purchase_order)
                          .where(:created_at => @date_start..@date_end)
                          .where(:total_cost => @cost_beginning..@cost_end)
  else
    @purchase_requests = PurchaseRequest.includes(:purchase_order)
                          .where(:created_at => @date_start..@date_end)
                          .where(:total_cost => @cost_beginning..@cost_end)
                          .where("purchaseorder.VendorRef_ListID = ?", @vendor)
  end

via Chebli Mohamed