In Drupal 6, if you know the taxonomy term ID, you can get the vocabulary ID by using the following code:

$term = taxonomy_get_term($tid);
$vid = $term->vid;

If you have a node ID, then you can use the following code to get the vocabulary ID of all the taxonomy terms associated with the node using the following code:

$node = node_load($nid);
$vids = array();

if (!empty($node->taxonomy) {
  foreach ($node->taxonomy as $tid => $term) {
    $vids[] = $term->vid;
  }
}

In Drupal 7, the code would be the following:

$term = taxonomy_term_load($tid);
$vid = $term->vid;

In Drupal 7, the node property $node->taxonomy doesn't exist anymore. Instead, there is $node->field_<vocabulary_name>, which is an array with two different structures.

• tags

  

• other taxonomy terms

  

Using field_get_items(), you would get the taxonomy terms in the language they would be displayed, or in the language whose code is passed as argument to the function.

$items = field_get_items('node', $node, $field_name);

$node contains the node object, and $field_name the name of the taxonomy term field.

$items contains a simplified array, compared to the array contained in $node->field_<vocabulary_name>.