APPLE BLOOM Avatar!! Sorry...
Anyways, I have some bettas in 1 gallons but they don't have heaters
I usually only use them when i get new fish or for treatment. I have had some pretty sad looking fish heal up nicely using a 1 gallon, unheated tank.
I don't like bowls or vases simply because its hard to get the betta out using a cup as they have small openings. If you can, i would go with a 1-3 gallon kritter keeper. With the 3 gallon (which I can get for $12 at petco) you can use a 25 watt heater :)
I'm gonna have to get them into something bigger before too long. When it gets -50F outside, it is barely 45 in here