If you're genuinely critiquing the security of this implementation of the Vigenere cipher - don't worry. The Vigenere cipher itself is known to not be very secure at all. Cryptalaysis and cracking it is piss easy (at least when the message and key isn't extremely long I suppose). Nobody should be using this to secure actual sensitive information ever.
Yeah, no, it was a silly comment on my part; I was just nerdsniped by an unnecessarily weak repeating-key cipher. :)
Valid critique, though, and I do appreciate it :-)
Because it’s intention is to be low barrier educational?
This is like asking a child doing writing exercises, “Why on Earth would you rewrite the same letter over and over again?”