Yoga is a great accompaniment to a good strength and cardio routine, but not a replacement for either.
Extending your flexibility will help with your range of movement and ensure you get the most out of your other fitness work.
But yoga itself doesn't burn very many calories (since it's based on staying still) and therefore doesn't directly contribute to weight loss very much at all by itself.
Of course, someone will probably come on and say they lost 40lbs in 10 weeks doing only yoga daily, so ... YMMV. Do your own research.
Deb, in New Zealand