sort.php

<?php

// Paste this code into your theme/functions.php
add_action( 'abrs_countries_dataset', function ( $countries ) {
	$top = [ 'VN', 'US' ];

	uasort( $countries, function ( $country1, $country2 ) use ( $top ) {
		$top = array_reverse( $top );

		$priority1 = array_search( $country1['alpha2'], $top );
		$priority2 = array_search( $country2['alpha2'], $top );

		$priority1 = ( false === $priority1 ) ? -1 : $priority1;
		$priority2 = ( false === $priority2 ) ? -1 : $priority2;

		return $priority1 < $priority2 ? 1 : -1;
	} );

	return $countries;
} );