Edit in JSFiddle

$(".vimeo-thumb").each(function(){        
        var id = $(this).data('vimeoid');
        $.getJSON('http://www.vimeo.com/api/v2/video/' + id + '.json?callback=?', {format: "json"}, function(data) {
            $('.vimeo' + id).css('background-image', 'url("' + data[0].thumbnail_medium + '")');
        });        
    });      
.vimeo-thumb {
    background: #fff url(data:image/png;base64,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)no-repeat center;
}
<img alt="Vimeo" class="vimeo-thumb vimeo43430508" data-vimeoid="43430508" src="data:image/png;base64,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">