Everybody has different tolerance to cold water. However since you are from Singapore and I'm from Indonesia, I assume our tolerance to cold water would be about the same.
Based on my experience diving in Galapagos (3x), I would say 5mm full wetsuit would not be enough, even with 5/3mm hooded vest over the 5mm wetsuit. If you have a new 5mm wetsuit, it might be OK for the first few days of the diving. The first day of diving around Baltra with water temperature ~ 17 °C (~ means +/- 1 °C) can be tolerable as your core body temperature is still warm & fresh. The next few days you will be in Darwin and Wolf, where the water temperature would be ~ 25 °C. You may still tolerate those water temperatures as they are warmer than Baltra, but your core body temperature starts to go down and you are getting tired. Then when you are in Cabo Douglas and Punta Vincente Roca with water temperature ~15 °C, you may start feeling cold and just want to get over the dive, even though you'll miss the Marine Iguana, Red-Lipped Batfish, Mola Mola, and Bullhead Sharks. If you have an old 5mm wetsuit, the thickness may not be 5mm anymore (perhaps it's down to 3mm). Then you'll be more miserable.
You can always rent 7mm wetsuit from the liveaboard, if you won't be diving in cold place, other than Galapagos.
My motto is, it is better being warm as you can always crack your hood around chin / neck / chess area to let some cold water in, than being cold with nothing to layer up with underwater.
I have 3mm gloves, which were fine then. I hate to get cold feet, so I wear 5mm boots with 3mm Neoprene socks.