Was katakana used in the teaching of Japanese as a foreign language?

Question

The Wikipedia article Japanese language education in Vietnam, says that during the 1940s, Japanese was taught in Vietnam using either romaji or katakana. Is it true that katakana, rather than hiragana, was used?

My assumption was that if only one set of kana were taught, then it would have been hiragana. The Wikipedia article on furigana mentions that native speakers of Japanese learn hiragana before katakana, and that furigana is more commonly written in hiragana than katakana. Likewise, Sayo Masuda, a native speaker of Japanese who wrote her autobiography in 1957, had learnt hiragana (as an adult) but not katakana or kanji.

Answer 1

From this this paper, in an extract authored by one Mr. Sekino (関野) in 1943:

　普及会に於ては日本語教育の課程を便宜上四期（各期三ヶ月）に分けて
次の要領に依つて教授して居る｡
第一期　「ローマ」字を用ひて正しき発音を教授し、「カタカナ」を用ひて
「ハナシコトバ」の教授を主眼とし、更に日本語の基礎文型を知らしめる｡
　教材として国際学友会編纂「日本語教科書」基礎篇を使用する｡
第二期　「ひらがな」を用ひ、教材として前記「日本語教科書」巻一中よ
　り適宜選択する。
　本期より漸次歴史的仮名遣を教授する。
第三期及第四期　夫々前記「日本語教科書」巻二、巻三中より教材を選択
　して教授する。

It's a pretty old usage of Japanese. I'd be grateful if somebody other than me could lend a hand in giving a proper translations of things like 用ひ (I assume 用い?) in this text, but I think it has the answer OP is looking for.

From what I understand, they did teach hiragana, but katakana first, and this was used in items such as childrens' books. Katakana was used as a gateway to Japanese grammar and pronunciation, and this was followed by the education in hiragana and kanji afterwards.

The reasoning for this could be down to katakana's greater status historically (such as being used in treaties with Korea and China in the late 1800s). Katakana was once historically the only script, and up until around 1900 katakana-kanji was used exclusively (including in treaties with Korea and China), but nowadays hiragana is by far the more useful of the kana syllabaries. Between 1900~1945, I know that hiragana was used informally (in letters between friends and in wartime posters), but I'm not sure when it was accepted into formal documents, if this was even pre-war.

As for Sayo Masuda, she never received any education in her youth, and post-war Japanese script reform made (and makes) starting with hiragana a more useful tool; probably why she and Japanese children nowadays learn it first. There's also the viewing of hiragana historically as "womens' writing", at least pre-1900s.