Hi Psyfreak,
Thank you for choose our theme.
1. You Can add filter in Appearance > Widgets > WooCommerce Filer Sidebar or WooCommerce Sidebar.
I’m not sure if plugin multi vendors support filter by vendor (which plugin you’re using for multi vendors)
2. Please contact to plugin to add option for user can follow vendors.
3. Please use this plugin and set permission for vendors user can use plugin Pin Maker.
Kind regards
Harry
Premium Wordpress themes and plugins, Best WooCommerce theme https://themeforest.net/user/janstudio/portfolio?ref=janstudio