How do you use the Venti Calendar plugin with the Element API plugin?
I don't have the Venti Calendar plugin, so I can't test this, but it should be as simple as setting the elementType propety to whatever custom element type the plugin has created and you want to use and (optionally) setting any criteria you want defined by the element type.

return [
    'endpoints' =&gt; [
        'news.json' =&gt; [
            'elementType' =&gt; 'Venti_Event',
            'criteria' =&gt; ['key' =&gt; 'value'], 
            'transformer' =&gt; function(Venti_EventModel $eventModel) {
                return [
                    'key' =&gt; $eventModel-&gt;property,
                ];
            },
        },
    ]
];