title: get vocabulary id by name
body: <p>I can retrieve a vocabulary id directly from DB,
  but is there a built in function for this?</p>
  
  <p>for example:</p>
  
  <p>i have a vocabulary called "listing", 
  i need that built in function takes "listing" as function argument, and return 
  a vid.</p>
  
  <p>i am using drupal 6</p>