Drush minimal bootstrap only for variables for vget
The general answer to your question is that you cannot change the bootstrap level of a Drush command as a user; Drush has already specified the minimum bootstrap level in each command's command record.  Running a command at a lower bootstrap than it needs will cause it to fail.

However, in the case of variable-get, it looks like you are correct.  vget is set to use a full bootstrap, but it would probably work just fine with DRUSH_BOOTSTRAP_DRUPAL_DATABASE.  This could be classified as a Drush (performance) bug.

If you would like to help out here, you could submit an issue to the Drush issue queue.  Add DRUSH_BOOTSTRAP_DRUPAL_DATABASE to the variable commands, test it and post a patch, and the maintainers would be happy to commit it.

Here's what the record for variable-get would look like:

  $items['variable-get'] = array(
    'description' =&gt; 'Get a list of some or all site variables and values.',
    'arguments' =&gt; array(
      'name' =&gt; 'A string to filter the variables by. Variables whose name contains the string will be listed.',
    ),
    'examples' =&gt; array(
      'drush vget' =&gt; 'List all variables and values.',
      'drush vget user' =&gt; 'List all variables containing the string "user".',
      'drush vget site_mail --exact' =&gt; 'Show the variable with the exact key "site_mail".',
    ),
    'options' =&gt; array(
      'format' =&gt; array(
        'description' =&gt; 'Format to output the object. Use "print_r" for print_r (default), "export" for var_export, and "json" for JSON.',
        'example-value' =&gt; 'export',
      ),
      'pipe' =&gt; 'A synonym for --format=export. Useful for pasting into code.',
      'exact' =&gt; 'Only get the one variable that exactly matches the specified name.',
    ),
    'bootstrap' =&gt; DRUSH_BOOTSTRAP_DRUPAL_DATABASE,       
    'aliases' =&gt; array('vget'),
  );