Autoloading for both WordPress classes and PSR2 files in a /vendor directory.

autoload.php

<?php
/**
 * PSR4 autoloader using spl_autoload_register
 * @package HN_Reactive
 * @author Thomas J Belknap <[email protected]>
 */

namespace HN_Reactive;

spl_autoload_register( function ( $class ) {
	$wordpress = wordpress_include( $class );
	$psr2      = composer_include( $class );

	if ( file_exists( plugin_dir_path( __FILE__ ) . $wordpress . '.php' ) ) :
		require_once( plugin_dir_path( __FILE__ ) . $wordpress . '.php' );
	else :
		require_once( plugin_dir_path( __FILE__ ) . '/vendor/autoload.php' );
	endif;
} );

/**
 * Format the incoming class name to the WordPress file naming standard.
 * @param $class
 * @return string
 */
function wordpress_include( $class ) {
	$parts = explode( '\\', $class );
	// Remove HN_Reactive index:
	unset( $parts[0] );
	if ( ! empty( $parts ) ) :
		$parts[ count( $parts ) ] = 'class-' . wp_class_to_file( $parts[ count( $parts ) ] );
		$class_path               = implode( '/', $parts );
		return $class_path;
	else :
		return false;
	endif;
}

/**
 * Format the incoming class name to the PSR2 file name standard.
 * @param $class
 * @return mixed
 */
function composer_include( $class ) {
	$class_path = preg_replace( '/\\\\/', '/', $class );
	return $class_path;
}

/**
 * Translates a WordPress-valid class name to a WordPress-valid file name (e.g. Class_Name - class-name)
 * @param    string   $str    String in camel case format
 * @return    string            $str Translated into dashed format
 */
function wp_class_to_file( $str ) {
	return implode(
		'-',
		explode(
			'_',
			preg_replace_callback(
				'/[A-Z]*?/',
				function( $matches ) {
					return strtolower( $matches[0] );
				},
				$str
			)
		)
	);
}