Ok I figured it out:
<?php
$posts = get_field('venue');
if( $posts ): ?>
<?php foreach( $posts as $post): // variable must be called $post (IMPORTANT) ?>
<?php setup_postdata($post); ?>
<?php $vmap = get_field('venue_google_map', $post->ID); ?>
<div class="acf-map">
<div class="marker" data-lat="<?php echo $vmap['lat']; ?>" data-lng="<?php echo $vmap['lng']; ?>"></div>
</div>
<?php endforeach; ?>
<?php wp_reset_postdata(); // IMPORTANT - reset the $post object so the rest of the page works correctly ?>
<?php endif; ?>
