Although you can get Exiled Lands Feats from completing the Vaults I don’t think it’ll be the main way of getting them on Siptah once it goes live.
At the moment “Spawn Elder Siege Things” is off by default in 2.3. Try turning it on and build a decent sized base in the Maelstrom (385+ building pieces). If you can build a thrall army strong enough to deal with what turns up you’ll see what I mean. Its still all subject to RNG but the odds of you getting something from the Exiled Lands are much greater than “maybe 1 per Vault run”. I’m getting 1-4 EL feat scrolls per 15 minute Maelstrom and a bunch of other things (I love the idea of powering my furnace with the heart of a demon!).
And that’s before you factor in what you can do with all those ???'s you’ll get…