Thus I pacified Psyche and kissed her, And tempted her out of her gloom— And conquered her scruples and gloom:
Finally, the speaker gets his way. He talks Psyche into seeing things his way, and calms her down. He gives her a kiss to comfort her, and encourages her to come out of her dark sadness ("gloom") and into the light. He overcomes her objections ("conquered her scruples") and gets her moving again.
And we passed to the end of the vista, But were stopped by the door of a tomb—
They roll along for a while until they get to the end of the place they could see ahead of them on the path (the "vista"). All of a sudden, something stops them. A tomb!
That's probably bad news. We were supposed to be walking into the light, into peace and heaven and all that good stuff. Now, all of a sudden, we're facing a tomb. Something's not right here…
By the door of a legended tomb; And I said—"What is written, sweet sister, On the door of this legended tomb?"
The door of the tomb is "legended." That's just a fancy way of saying that there's writing on it.
Our speaker, not being too swift on the uptake, asks his soul (his "sweet sister") what's written on the tomb.
She replied—"Ulalume—Ulalume— 'Tis the vault of thy lost Ulalume!"
Here it comes – the big reveal. Psyche knows exactly what's written on the tomb. It's the "vault" (another word for tomb) of the speaker's "lost Ulalume."
Up until now, we didn't know who or what this poem was named after. Now, it turns out that it's the speaker's dead loved one. His girlfriend, his wife, his sister? No way to know, but we've got a hunch she's the reason he's feeling so lousy.
One other thing: Notice how Psyche repeats that name, and how smooth and echoey it is? Ulalume! Doesn't it just sound like the definition of grief?