A bad catalytic converter will not cause your Saturn
Vue to stall. You never did tell us what codes are stored in the computer.
There are code that some mechanics think is caused by a converter, but really are not. If you had a P0420, then yes. Some cheap aftermarket parts do not work well enough and the code can return.
For the stalling out, try cleaning the carbon from the throttle. This is common for dying out whn coming to a stop, idling or do parking lot turning.