The trick involves placing a transparent image as the background cover, and adding a background image to the video object with a background-size of cover.

<style>
	video { background: transparent url('<?php the_field('video_icon'); ?>') 50% 50% / cover no-repeat; }
</style>
<video width="100%" height="100%" controls poster="<?php bloginfo('template_url'); ?>/img/transparent.png">
	<source src="<?php the_field('video_file'); ?>" type="video/mp4">
</video>