Drupal - Where do I find the vocabulary ID ($vid)?

Actually you can use:

$vocab = taxonomy_vocabulary_machine_name_load('my_vocabulary');
$vid = $vocab->vid;

Following Manu, if you have command line Drush access, you could do:

drush php-eval '$tax=taxonomy_vocabulary_machine_name_load("main_site_structure");echo $tax->vid;'