There are flights on sale via Japan and Korea, and also some brand spanking new direct flights SGN-SFO-SGN on VN. These are operating already, but independent tourism isn't open yet (i.e. you can't get a visa unless you're coming on a package tour). Various sources say March, April or May for independent tourism reopening. I would suggest waiting until you hear about some people actually doing it, as the government here is quite reactionary and things can change at short notice.