Play games
Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
If you're going to have Fallout: New Vegas Ultimate Edition be a reward, I expect that Fallout 3 should have the same treatment. I don't need any other game and I'd be happy if I could take advantage of my gems in a form that I'd appreciate.