The intelligence failings fall into three broad categories. In Vietnam, the US military possessed a lack of foresight, neglected fundamental intelligence-gathering principles, and suffered from an absence of strategy and direction.
On a fundamental level, US military intelligence was unable to gauge the level of communist infiltration in the South of the country. This meant the US forces had almost no idea of the size of the enemy’s forces.
It wasn’t until 1967 that the CIA appointed anybody to study the actual Vietcong numbers. A glaring neglection of the fundamental principles of intelligence gathering.
Nowhere was the American intelligence community’s lack of foresight more obvious than in the build up to the Tet Offensive in 1968. In a report compiled following the attacks, the CIA, NSA and Joint Chiefs of Staff (JCS) admitted “few US or GVN [Government of South Vietnam] officials believed the enemy would attack during Tet”. 
But why? There was already a precedent for the North Vietnamese forces striking an enemy on the annual Tet holiday. In 1789, Vietnamese forces attacked and defeated the Chinese army during Tet celebrations, driving them out of Vietnam.
Senior members of Johnson’s administration, including Secretary of Defense Robert McNamara, who were kept abreast of the latest intelligence reports from Vietnam, admitted that they were astonished that the communists possessed the military capability to mount an offensive on the scale seen at Tet. There was a significant gap between the reality on the ground in Vietnam and what the intelligence community in Washington believed. This was entirely due to intelligence failings.
The entire intelligence operation lacked direction. This is best illustrated by the lack of Vietnamese speakers among the CIA officers in the field.
Language and communication skills would seem like a basic pre-requisite for an effective intelligence-gathering campaign, but in 1967 there were only three CIA officers in the field with Vietnamese language skills.
The rest of the CIA officers based in the country relied on interpreters, who themselves were open to Vietcong infiltration.