Allow sub-site admin to use editing features

You might be able to do that by using the vac_required_capability filter to change the capability that is required to access the plugin to something that subsite admins have. I think the manage_options capability would work.

For example:

function ws_change_vac_capability($cap) {
    return 'manage_options';
}
add_filter('vac_required_capability', 'ws_change_vac_capability');

functions.php would work. I would not recommend adding the code to existing plugin files.

Alternatively, you could create a mini-plugin that just runs this code, or use a plugin like Code Snippets (no affiliation) to run the code without modifying any files.