Does a student need to know basic vowel sounds?

My students generally pronounce words in romaji/katakana.

I try to explain that it like this to them(usually in more detail):

a(hat), like a sheep crying "baaa"

e(bet), like え but the mouth is more open

i(hit), like え but the mouth is more closed

o(hot), like right in between あ and お

u(hut), like あ but the mouth is more closed

But either my explanation isn't good enough or they're too used to romaji pronunciation. I’m not sure how much I should focus on this. Should I explain it once and then move on? Or should I gradually train them to hear the differences over time? I feel like understanding these vowel differences would make English much easier for them in the long run. At the same time, though, focusing too much on pronunciation early on could discourage some students if they feel English is too difficult.

On the other hand, a student who continues learning English with katakana pronunciation for years is probably still better off than a student who quits entirely. The one's I mentioned are only just 5/20 vowels. And not to mention consonants like l, r, th, and v.