Between Wiktionary and Etymonline, the suggestion is the word stems from Latin Vesi as a Latinized form of their tribe name, itself coming from PGmc *wesuz "great, excellent".

I really hope this is right, because a nation of goths who call themselves "the Good Guys" is hilarious to me.

i gathered that it wasn't cassiodorus who started using the term that way but a later writer. he invented the term but just called the ostrogoths "goths"