How about somewhere else in the Philippines like Cebu? There are probably flights from TPE to other places besides Manila in the Philippines. Just suggesting that because I don't know a single person who has particularly liked Manila.

Saigon isn't bad, and if you're there 4-5 days, I'd maybe look into a quick trip to the Mekong Delta too. On the other hand, you've already been to a couple cities in Vietnam, both of which I found to be more pleasant than Saigon.