Thank you very much Lynne,
I had to add a little bit more and actually change the plugin code to a simpler version.
Inside the plugin I had to create a third variable $new_banners and only pre-registered that variable within template and ran it through parse_templates hook
To make it work, I just dropped this code
{vb:raw new_banners} on the template and now it works just as before under vB3.x.
For some (unknown) reason, {vb:raw random.banner.random_number} wasn't working as you suggested, although the arrays were set correctly.
I appreciate your help and
borbole for helping me through this.
I hope other users find this little experience useful in their sites.