You would do it as before, only use the new syntax. So, something like this:
PHP Code:
$pagetitle = 'My Page Title';
$foo = 'hello';
$bar = 'world';
$templater = vB_Template::create('first_template');
$templater->register('foo', $foo);
$templater->register('bar', $bar);
$my_variable = $templater->render();
$templater = vB_Template::create('TEST');
$templater->register_page_templates();
$templater->register('my_variable', $my_variable);
$templater->register('navbar', $navbar);
$templater->register('pagetitle', 'Test Page');
print_output($templater->render());
You would use {vb:var foo} and {vb:var bar} in your first_template to insert the variables $foo and $bar and then you would use {vb:var my_variable} in the TEST template to insert the output from $my_variable.