Until a security system is installed in September, veterans of the Vietnam War plan to guard a Sacramento memorial that has been vandalized over the last few months. Pairs of volunteers, some of them attired in camouflage fatigues they wore in Vietnam, will stand guard in four-hour shifts at night and on the weekends. State police and members of the California Conservation Corps will keep watch during weekdays. The $2-million memorial to more than 5,800 Californians killed in the Vietnam War was dedicated in December at the east end of Capitol Park. Vandals broke parts of an M-16 rifle sculpted in bronze and a miniature M-16 on a wall plaque in late February and again in April. A flag was also stolen in April.