Reference Overlay Example

<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title></title>
  <script src='//api.tiles.mapbox.com/mapbox.js/v1.4.0/mapbox.js'></script>
  <link href='//api.tiles.mapbox.com/mapbox.js/v1.4.0/mapbox.css' rel='stylesheet' />
  
  <style>
    body { margin:0; padding:0; }
    #map { position:absolute; top:0; bottom:0; width:100%; }
  </style>
</head>
<body>
<div id='map'></div>
<script>
var map = L.mapbox.map('map').setView([44.2594312, -72.57705688], 15);

// Add a dynamic or external datasource - from WMS in this case
var orthoWMS = L.tileLayer.wms("https://maps.vcgi.org/arcgis/services/EGC_services/IMG_VCGI_BW_WM_CACHE/ImageServer/WMSServer", {
    format: 'img/png',
    transparent: true,
    layers: 1,
    attribution: 'State of Vermont'
}).addTo(map);

// Place the Mapbox reference layer in the Leaflet topPane
var topPane = map._createPane('leaflet-top-pane', map.getPanes().mapPane);
var topLayer = new L.mapbox.tileLayer('landplanner.map-m2tg9ziq', {
	maxZoom: 18
}).addTo(map);
topPane.appendChild(topLayer.getContainer());
topLayer.setZIndex(7);

</script>
</body>
</html>

https://api.tiles.mapbox.com/mapbox.js/v1.4.0/mapbox.js