Hello,
In the email preload, you can access to the $mail object which contains the different values for the email address and the name.
You can modify these values in the preload and they will be used afterwards when the email is sent.
$mail->dst_name = @$data->cart->billing_address->address_firstname;
If you want to sign with the vendor name, it will be more complicated because you're in an HikaShop email (so in the HikaShop context) ; the order is the customer order and not the vendor one, so the vendor information cannot be reach like that (specially if the order can have products of several vendors in the same time).
Regards,