I can't imagine keeping a grape bonsai indoors and expecting it to survive long-term. Site your tree to receive several hours of early morning sun and afternoon shade--that's it
And let it have all the rain you can throw at it, while you're at it
You can feel good about keeping it healthy for as long as you did--it proves you'll be able to do even better once you follow the advice given by others and myself to take the plunge and venture outdoors with your bonsai. You'll never look back
And remember this...you can always bring it indoors for an evening to enjoy. Or a couple of days for a club show. The best of both worlds, I'd say.