Using BuddyPress Message system in other parts of your WC Vendors Site

gistfile1.php

<?php
/* Buddypress Private Messaging Mod  */

/* 
	Usage : 

	Put this in your templates to create a message link you can then style

	templates this could go in : 
		*  description.php (put the message link in the end of the description)
		*  content-singple-product.php (place this message above the item details, if you combine this with a small vendor header)
		*  order-details.php (allows them to message about an order) 
		*  archive-product.php (Put a vendor header in the per vendor product pages)

	if ( function_exists('bp_is_active') ) { ?>
		<a href="<?php echo bp_custom_get_send_private_message_link($product->post->post_author,'Subject here','Predefined message Information');?>">> MESSAGE SELLER</a>
	<?php } ?>

*/

//  Change the Buddypress Search Slug to stop search collision. 
define( 'BP_SEARCH_SLUG', 'searching' );

function bp_custom_get_send_private_message_link($to_id, $subject=false, $message=false) {

//if user is not logged, do not prepare the link
if ( !is_user_logged_in() ) return false;

		$compose_url=bp_loggedin_user_domain() . bp_get_messages_slug() . '/compose/?';
		
		if($to_id)
			$compose_url.=('r=' . bp_core_get_username( $to_id ));
			if($subject)
			$compose_url.=('&subject='.$subject);
			if($message)
			$compose_url.=('&content='.$message);

		return wp_nonce_url( $compose_url ) ;
}

/* 
	Auto populate the message content
*/
add_filter('bp_get_messages_content_value','bp_custom_message_content_value');

function bp_custom_message_content_value($content){

	if(!empty ($content)) return $content;
	$content=$_REQUEST['content'];
	return $content;
}

/* 
	Auto Populate subject

*/
add_filter('bp_get_messages_subject_value','bp_custom_get_messages_subject_value');

function bp_custom_get_messages_subject_value($subject){

	if(!empty($subject))
		return $subject;

	$subject=$_REQUEST['subject'];

	return $subject;
}

?>

This should go in a file called bp-custom.php in your wp-content/plugins/ directory