In Vietnam, I prefer Vietnam Airlines but will also fly Vietjet without hesitation if there is a big price gap. I usually start with skyscanner, but that site seems not to always get Vietnam Airlines promos, so I will always double check VN's own website. Morning flights have a better chance of being on time. I do not fly on Jetstar.

SGN to CEB does not have any direct flights for now. Cebu Pacific will almost certainly be your cheapest option. SQ or CX could be booked with points. PR is also very nice for this route, but a bit more expensive than Cebu Pacific, sometimes more than double. You might see if UA still allows a stopover on a RT within Asia. If so, 3 stops isn't bad value, but I've not used them for a few years.