The English language has a great amount of borrowings from French. But why aren't such letters as "ç"(façade) and "é"(café, protégé) changed if they don't exist in the English alphabet and there are "c" and "e" only? Also I have a question about the surname Brontë(I mean Charlotte Brontë). Again English doesn't have a letter like this (ë). I looked up the etymology of the surname. It told that the word "Brontë" derives from Ancient Greek and initially meant "thunder". But "ë" looks like a French letter, though Charlotte Brontë didn't have French roots if to believe Wikipedia and was born to British parents. So what is French-like letter doing in the British surname of Greek origin?
Foreign graphemes are indeed replaced with their closest bare-Latin alphabet equivalents, thus facade, cafe. Some people may elect to retain source-language spelling conventions. The probability of doing that depends on many things, such as what language the word or expression comes from, how prevalent borrowings are from that language, plus the probability that a given writer knows the foreign spelling. Vietnamese words are very low frequency in English and typically the diacritics are stripped off (and probably spellings like bánh mì, phở, Nguyễn are only produced by Vietnamese speakers, because non-speakers have no idea what those marks are about). Among English speakers, there is greater awareness of French spelling; somewhat less knowledge of German; relatively little for Italian.
The details of Brontë's name would be batter pursued on the English or History SE, but the explanation that it was needed to indicate that the name is bisyllabic is credible enough in terms of linguistic facts.
Recent English borrowings from French like façade or café or naïve do generally retain their diacritics in English, and most speakers of English at least on this side of the Atlantic would consider these to be the correct spelling. But the great majority of the French loan words in English were borrowed from Middle French. At that time the orthography of French was not firmly established, so English scribes wrote the French words more or less as they heard them.