Forum unusable. Where to use SET NAMES? Or do you have any other suggestions?

[ringleader]

There's a few things around this issue that I've seen.
Googling, and cut and paste, gives:

You need php 4.4+ and apache 2.0.x to properly support vBulletin in utf8 format

    * Perform this query with a MySQL client like phpMyAdmin.

      ALTER DATABASE `db_name` CHARACTER SET utf8;

      Where db_name is the name of the database.
    * The collation will be set to the default: utf8_general_ci
    * Open your config.php and find:

      // $config['Mysqli']['charset'] = 'utf8';

    * Make sure it looks like this:

      $config['Mysqli']['charset'] = 'utf8';

    * Save config.php and upload it.
    * Start installing your vBulletin as usual, and don?t forget to update your languages HTML charset via ACP.

Final note: MySQL versions prior to 4.1 don?t support charsets and collations.

Converter for 3.6.4+ 
https://vborg.vbsupport.ru/showthread.php?t=141591

Description: This script converts the database of vBulletin forums (from and to) any of the charsets that are supported by iconv. also it has the ability to convert the tables of hacks that you had installed.

vbulletin emails are sent with utf-8 encoding.
This hack made for  3.6.4 might fix this:
https://vborg.vbsupport.ru/showthread.php?t=144040

Hope that helps