I know that in Vietnam, gun trucks with improvised armor were created as an emergency measure for convoy protection early on. Then the V-100 armored car was introduced to Vietnam. So, did the armored cars replace or supplement the gun truck, or what happened?

The M703/V100s were supplemented by the unit-built guntrucks. The issue was that there were not a lot of M703s since they were only fielded to MP units. Transpo units developed the guntrucks to help protect their own convoys since there were only a couple MP escorts per convoy.