How to create a meta_query to get all posts with a specific meta data?

If you want to target meta key vendor and meta value farsi, you need to pass the following meta_query arguments:

$args = array(
    'posts_per_page' => -1,
    'post_type' => 'product',
    'meta_query' => array(
        array(
            'key'     => 'vendor',
            'value'   => 'farsi',
        )
    )
);

Also to note, as Jiten highlighted, your meta key may be wpcf-vendor.