The world includes Vietnam! So, the other letters 'j, f, w and z' are now OK to use in many places - on billboards, posters, T-shirts... But, if you work for any type of Vietnamese authority or association, then limit their use until the authority declares them as acceptable letters. But, the other alphabets that are used in English might not be accepted. For example: the Greek alphabet is very useful for English speakers - but in Vietnam it can lack meaning - it would be a decoration for the general population. Educated people will know more letters from the Greek alphabet.