WordPress Rewrite

I’m trying to get http://example.com/site/vendors/?u=abc to rewrite to http://example.com/site/vendors/abc but I can’t for the life of me figure out how to get the url rewrites to work.

/site/ is the root directory for my WP installation
/vendors/ is a page

assuming u is a custom query var, you have to first add it to the array of recognized query vars:

function wpa_query_vars( $query_vars ){
    $query_vars[] = 'u';
    return $query_vars;
}
add_filter('query_vars', 'wpa_query_vars');

Then add an internal rewrite rule that accepts anything appended to vendors and passes that as the u query var:

function wpa_rewrite(){
    add_rewrite_rule(
        'vendors/([^/]+)/?$',
        'index.php?pagename=vendors&u=$matches[1]',
        'top'
    );
}
add_action( 'init', 'wpa_rewrite' );

Make sure to flush rewrite rules once for this rule to be added, you can also do this by just visiting the Settings > Permalinks admin page.

Then in the template, you can access the value of u via get_query_var():

$vendor = get_query_var( 'u' );