There are two'flights' a day. One is operated by VN (Vietnam Airlines), and carries both CX codeshare. This will not be eligible for a BA miles redemption.

There is also the KA295 which is a KA flight on KA metal and also carries a CX code. This is eligible for BA miles redemptions. A cursory glance at loads indicates this flight is rather popular, which may be why there are no seats available now.