Sorting posts by ACF field

You are not going to this in one query, you will need to do two queries. The first query will hold the posts which will hold the meta key, the second will be the posts without the meta key.

(Just a note: never use query_posts unless you intentionally wants to break things)

You can try something like this

$args1 = array(
    'orderby' => 'meta_value',
    'meta_key' => 'prempost',
    'order' => 'ASC',
);
$query1 = new WP_Query($args1);

//Do your stuff here

$args2 = array(
    'meta_key' => 'prempost',
    'compare' => 'NOT IN'
    'order' => 'ASC',
);
$query2 = new WP_Query($args2);

//Do your stuff here

You can also maybe try to get all the posts at once and then sorting them with php using usort()