Dokan Marketplace store link in single product page

Shortcodes should only ever return content, never echo or printf() it.

From the add_shortcode() documentation:

Note that the function called by the shortcode should never produce an output of any kind. Shortcode functions should return the text that is to be used to replace the shortcode. Producing the output directly will lead to unexpected results.

So your code should look more like this.

add_shortcode('vendor_shop_name', 'vendor_shop_name_function');
function vendor_shop_name_function() {
    global $product;
    $seller = get_post_field('post_author', $product->get_id());
    $author  = get_user_by('id', $seller);
    $vendor = dokan()->vendor->get($seller);
    $store_info = dokan_get_store_info($author->ID);
    
    if (!empty($store_info['store_name'])) {
        return sprintf( '<span class="vendorpage"><a href=" %s">in this store %s</a></span>', $vendor->get_shop_url(), $vendor->get_shop_name() );
    }
    // Nothing found; return an empty string.
    return '';
}