It looks like CX has made some additional schedule changes, as my (waitlisted) flight in early March no longer offers F. CX cleared me into J, and sent an email asking me to contact them. I chatted with them on the website, and was told accepting the J flight or taking a refund were my only options.
I called in a bit later and was told they would email a different department to try to move me to later date where F is offered (and could be confirmed). Interestingly, they also asked whether I had approval to enter VN (I would be traveling as a tourist).