Because Vietnam, unlike the rest of mainland SEA, was influenced by China a lot more than it was influenced by Hindu-Buddhism. A good example of this is the contrast between Dai Viet, which was the cultural core of modern Vietnam, and Champa, the Hindu-Buddhist mandala states that used to exist in central Vietnam.

Historically, only northern Vietnam (which later became Dai Viet state) and central Vietnam (which later became Champa states) among the mainland SEA region were ever under direct Chinese rules. Northern Vietnam was held under direct Chinese rules for 1000 years, while central Vietnam was held under direct Chinese rules for some 200-300 years. As a result, northern Vietnam was a lot more Sinicized than central vietnam post-independence.

In addition, the timing also helps explain the difference between Dai Viet and Champa. Northern Vietnam stayed under Chinese rule during the entire period of Hindu Brahmin's influence in SEA, while the proto-Champa states got independence just in time to receive Hindu-Buddhist influence and subsequently de-Sinicized itself to become Hindu-Buddhist states using Brahmic scripts. In contrast, after independence, Dai Viet in northern Vietnam chose to further self-Sinicized and adopted more Chinese influence, including the system of governance, fashion, and Chinese-language literature. It wasnt until Dai Viet annexed central Vietnam and southern Vietnam that all of Vietnam became Sinicized like it looks like now in the modern day.

When people talk about "Vietnam historically using Han scripts", they're talking about the cultural core in the north, the unified Vietnam under the Nguyen dynasty, and the ethnic majority Kinh. In contrast, Champa was historically Hindu-Buddhist like the rest of SEA, with Cham Vietnamese still using Brahmic script (and Arabic script among the Islamic Cham Vietnamese) in the modern day.