This is usefull, if you want to have same vid across multiple user devices once user is logged. Just have in mind vid value cannot be longer than 50 characters. In JS to keep same VID the same always for logged visitor JS could look like

Set VID if user is logged.

LHCChatOptions.callback = {};
LHCChatOptions.callback.loaded = function(inst){
inst.setVid('<?php echo md5($user_id .  $user_email)?>');
}; 

You can also set VID manually. System will detect old VID if it exists and map old settings to provided VID

<input type="button" value="Set VID" onclick="lh_inst.setVid('<?php echo md5($user_id .  $user_email)?>')"></input> 

How to merge using REST API?

$response = $LHCRestAPI->execute('setnewvid', array(
'vid' => '42rsfye2cwolnu3vst', // Old VID
'new' => 'lbv6mnwc2jg4mmxmx5w', // New VID
), array(
), 'POST');

See example here