What is the meaning of dome of a vast sepulcher?

That would be the circular or oval ceiling of a large structure like a cathedral.

sepulcher is a tomb, grave, or burial place.

From the Latin 'vastus', immense, empty; vast means of a great area or number, lesserly of, or referring to emptiness, thus; A 'vast' forest could be full of trees, or, 'vast' desert, of emptiness.

'Teapot dome scandal' means a government scandal involving a former United States Navy oil reserve in Wyoming that was secretly leased to a private oil company in 1921; became symbolic of the...

Vast means a large, empty space or distance. So some word would be BIG and EMPTYAnother word could be immeasurable.