I understand it this way:
There are many countries in Southeast Asia, and your questions should be answered according to their cultures, traditions, ethnic groups and other factors.
There are 11 countries in Southeast Asia: Vietnam, Laos, Cambodia, Myanmar, Thailand, Malaysia, Singapore, Indonesia, Philippines, Brunei and East Timor.
First of all, Southeast Asia is a gathering place for Chinese, and the Chinese character culture in Southeast Asia is relatively well maintained. Most Chinese in these countries know Chinese characters, including Malay, Indonesia, Philippines, etc. (This is a big difference between Chinese in the West. One of the special cases is Singapore, which is a country dominated by Chinese, and the Chinese education system has been preserved and constantly updated so far, so the vast majority of Chinese in Singapore certainly know Chinese characters.
next, Vietnam. Compared with other languages in Southeast Asia, Vietnamese and Chinese are closely related. There are many (6-7%) Chinese-Vietnamese characters in Vietnamese, such as: quoc gia (country), gia dinh (family), tinh cam (emotion), etc., but this does not mean that they know Chinese characters. Most people only know some Chinese characters with profound cultural significance, such as some festive ones: happiness, double happiness, longevity and so on; Another example is benevolence, righteousness, forbearance, wisdom and so on (the latter is limited to people with higher cultural level). This is related to the influence of Chinese culture on Vietnam.
In addition to the above words, only those Vietnamese who study Chinese and Confucianism or Chinese calligraphy in the old days know Chinese characters, while other Vietnamese don't. However, due to the close relationship between Vietnamese and Chinese, Vietnamese people have much advantages in learning Chinese compared with other countries, especially westerners. (Note: Chinese, not Chinese characters) They are the same as Japanese and Koreans in this respect. Han, He, Han and Yue are also the cultural circles of Chinese characters (among them, the Japanese still use Chinese characters, so only they have the most advantage in learning Chinese characters).
In the end, the rest of Southeast Asian people don't know Chinese characters at all, and because their mother tongue doesn't have much contact with Chinese, it is also very difficult for them to learn Chinese. As compared with Europeans and Americans, they may have a slight advantage in learning Chinese, because the gap between the customs and cultures of eastern nations is not as big as that between eastern and western nations, so it is easier to understand each other's cultures, including languages. However, if we only talk about Chinese characters (again, Chinese and Chinese characters are two concepts), it will be difficult for all people (including Vietnamese) who are native speakers of phonetic languages to learn Chinese characters.
I hope my answer can help you, thank you!