Read Vietnamese history books, they had great difficulty colonizing small ceded Champa provinces: Viets didn't want to leave their hometowns; and migrant ships capsized due to storm, creating panic among unlucky ones about to be sent south. etc.
The problem is, Southeast Asia during colonial age was too sparsely populated to have a craving for new land, and the Philippines was too far for a hostility to develop.
(Having said that, if Viet Nam Cong Hoa unified the whole peninsular with war butterflied, Vietnam developed into an economic power like OTL South Korea, Philippines might already be under de facto economic occupation by Vietnamese investors.)