By Khanh Vu HANOI (Reuters) -Vietnam and the Philippines agreed on Tuesday to boost cooperation among their coastguards and to prevent untoward incidents in the South China Sea, in an announcement during a state visit to Hanoi by President Ferdinand Marcos Jr. The two Southeast Asian countries have competing claims over some parts of the South China Sea, a conduit for $3 trillion of annual ship-borne trade that China claims almost in its entirety. The two memoranda of understanding on security covered “incident prevention in the South China Sea” and “maritime cooperation” among coastguards, ac…