It could’ve gone two different ways: Indianization (Thailand, Laos, Cambodia) or Sinicization (Japan, Korea).
Without Chinese occupation, Vietnamese might have been influenced by the southern Indianized kingdoms became Indianized itself, and Champa wouldn’t have been wiped out. In later periods, it might’ve been Islamized by Maritime Southeast Asia just like Champa, or it could’ve stayed Indianized like other Mainland Southeast Asia, sharing cultural heritage with Thailand, Laos, and Cambodia.
Language-wise, majority of loanwords would have been from Indic languages, and their script would’ve long been the phonetic abugida script, sharing similarities and origin with Thai, Lao, and Cambodian languages.
However, it could’ve also been Sinicized either way. Several Đông Sơn drums created before Chinese occupation in northern Vietnam were found with Chinese characters on them. This could’ve meant that Vietnam, whether it was conquered or not, was on the verge of willing Sinicization like Korea and Japan. Therefore, it might’ve just ended up like today, but has less similarities with China (since it wasn’t occupied, the influence wouldn’t have been huge, and Vietnam would’ve retained a lot more of its original culture, such as tattoos and hair-cutting).
Language-wise majority of loanwords would still be from Sinitic languages, just not as much as now. Without French colonialization, Vietnam would’ve still retained its Sinitic writing system, and at the dawn of Westernization that favored widespread literacy, would’ve standardized chữ Nôm completely, allowing mass education.