All you would need to do, is upload your error page to your web folder, from the VIP+ Hosting Control Panel, choose the 'Set-up Error Pages' button. From here you will be able to select the page you want for each type of error.
You can add your own .htaccess file so you can create a custom 404 page - just make sure you have a 'real' FTP client to manage things (I like Filezilla).
NB. Phuzzy (another Freeola member) created this guide you may find handy.