I'm remodelling my bathroom and want to add a light above the vanity that operates off the same existing switch that operates the overhead light.

The power current comes from the ceiling fixture to the switch.

I added the vanity light along the cable on its way to the switch. However, now when I operate the switch one light turns on and the other truns off.

Any thoughts? Thanks.

The cable runnig from the light to the switch was two (damage control) the rest of my answer was so totally wrong I had to delete it so no one would try it. Apparently at a certain age one loses the ability to talk on the phone and use the computer at the same time. Sorry Shank and others, go with Anthony.

I'm not sure I got that.

Currently, he black and the white are running from the ceiling fixture to the switch.

I pigtailed off both the black and the white wires to add the vanity light between the ceiling fixture and the switch.

Was this incorrect?

Yes, you are tied to the hot and switch leg going to the switch. You have no neutral for your vanity light, even though it's a white wire there, it's not a neutral. You can tie to the light fixture, and pick up your switch leg and neutral there.

Anthony's answer is correct, but I want to put it a different way. You need a hot when the switch is on and a neutral connected to the second light. Easiest way is to parallel it from the existing light, hot to hot, and neutral to neutral. The switch loop doesn't have a neutral at all, just a hot and a hot when the switch is on. So to get a neutral, you must start at the first light.