My daughters have friends who kept a betta in about a .5gal vase for 2 years. I didn't want to come across as a PETA member, so I half-heartedly suggested a larger tank if possible, but if that wasn't going to happen then at least do plenty of water changes (like 100% every other day) and I gave them a good supply of my NLS pellets. They ended up doing about 100% w/c twice a week, but did keep using the NLS. Surprisingly, it grew to be one of the largest betta I have ever seen.
I'm not condoning this........................but it is, what it is.