Hello,
I'm working on integrating vtiger with roundcube too, I have found the third party integration called VGS global, but their autocomplete system is just like what you said (autocomplete can fetch emails from all user not only the user who's logged in). To solve this go to the roundcube plugins folder, you'll find a plugin called vtcontacts inside its folder open the file vtcontacts_backend.php in this file
// $result=$rcmail->db->query("SELECT accountid, accountname, email1 FROM vtiger_account
// WHERE accountname like '%$value%' OR email1 like '%$value%'
// AND email1 !=''");
as you can see the query didnt check for the current user, here's the query i'm using but I have made some web services modifications to get the current_user
$accounts_query = "SELECT vtiger_account.accountid, vtiger_account.accountname, vtiger_account.email1 FROM vtiger_account
inner join vtiger_crmentity on vtiger_crmentity.crmid = vtiger_account.accountid
inner join vtiger_users on vtiger_users.id = vtiger_crmentity.smownerid
left join vtiger_groups ON vtiger_groups.groupid = vtiger_crmentity.smownerid
WHERE vtiger_crmentity.deleted = 0
AND vtiger_users.user_name = '$current_user_name'
AND vtiger_account.accountname like '%$searchQuery%'
OR vtiger_account.email1 like '%$searchQuery%'
AND vtiger_account.email1 != ''";