Lesenterbang,
I'm fully aware of who control over South China Sea, before VPK or TAXUL. But with good coordination between KUL and SIN ATC, plane can be cleared to intermediate level ie, FL310/290 before VPK/TAXUL instead of maintaining FL380/400 which well passed the TOD point once passed VPK/TAXUL fix.
Like many ATC in this world, normally the adjacent controller will cleared you to descend to an intermediate level and then pass it over to the next controller. One example is Wien (Austria), where most part of the descend from the east is controlled by Bratislava due to close proximity of the two FIRs.
As I have suggested earlier, the intermediate level (FL310/290) would probably a good level to fly over VPK/TAXUL before your next descend once KUL takes over. It won't be too high at that point that one would have no choice but to use speed brakes or charging with higher speed to get back on profile. It also depend on the type of plane you fly and speed at the time you cross VPK/TAXUL. I've flown the A330 and B777. This planes are much "cleaner" than the B737 and to loose height or speed, would most likely have to deploy the speed brake if higher than normal profile.
After all, it's up to individual preference. To descend to intermediate level or keep your initial cruising level and charge down with high speed and speedbrake. I don't fly to KUL nowadays as a "skipper". Just giving my opinion on the subject.