I have tried to bring them indoors with little success. They seem to not thrive indoors. Mine all shrivel up and die. I havwe tried watering them at different levels and different light levels and nothing seems to keep them alive.
But I have a theory that they really miss the humidity from outdoors and if you have a window that gets a lot of really warm almost direct sunlight, like those little plant windows some people have in their kithcens that actually stick out from the house and a generous amount of water that they may survive until fall and you can put them outside again.
But regardless it is worth a shot because about mid june in most areas violas are little shriveled up memories of a pretty plant.
If you let them seed out they are usually prolific self-seeders and will have new plants in the spring. My self-seeded plants tend to be a lot stronger than the original and last a little longer outside.
Hope I was helpful,
Beth