Evergreen should survive in the ground, don't know about grow bags

If I transplant them into the ground vacated today by the tomatoes, will they live all winter, or just die and come back in the spring?

Ditto Farmerdill's comments. Planted outside, the onions would die back, and remain dormant until Spring - provided the roots were established. Here, their winter hardiness was "iffy" in the ground without protection; even in your slightly warmer zone, they would probably not survive transplanting this late in the season.

I doubt they would survive in a grow bag exposed to outdoor temps, but they might make it if the bag were brought into a somewhat protected environment, such as an unheated garage. If the garage drops below freezing, the plants would stop growing until the temperature warmed. You would need to keep an eye on the grow bag, to ensure the soil doesn't dry out completely.

You could keep the onions going under lights, provided the light is strong enough... but I doubt the plants would be vigorous enough, or plentiful enough, to allow much of a harvest. It would at least ensure their survival until next year, though.

Evergreen Bunching is pretty easy to overwinter and propagate. You could split the bunch if big enough and try to plant some outside if you still have a good clump and save some inside. They won't be very edible outside when there is a deep freeze, but the roots will probably grow again in early Spring. Easy to grow, but fresh growth is better eating quality so you need to divide in the Spring or start with a new batch.

I ended up bringing them all in, some in a pot, others in a glass of water. All continue on, so far.