Yoga builds muscle while balancing it out with flexibility. So if you want to get more toned, it depends on what results you are looking for.
If you want to look like a body builder, yoga isn't your best option. When you go to the gym and lift weights, you are isolating a certain muscle. If after a couple of reps you don't stretch, the muscle fibers heal close together. This gives you the body builder aka 'bulky' look.
However, if you want to look sexy and elegant in a short sleeved dress, yoga will give you the long and toned arms you are longing for.
When you are moving through a vinyasa, your body is doing isometric poses. The muscles are being activated and elongated at the same time!
And who needs a gym and weights anyways if you can use your own bodyweight and get great results?
Off course it all depends on your personal goals which form of exercise you choose. The best gift you can give your body is to move daily and enjoy yourself!