1 Nephi 12:18

In this section, Nephi speaks more about his vision and the large and spacious building that his father saw. He says that the interpretation thereof is that it represents the pride of the world. It also represents the vain imaginations of the world. Pride of the world is, I believe, straightforward enough, but what could "vain imaginations" mean? It probably could be a lot of things, but I think that the dictionary can help. Sometimes the dictionary does not really help with scriptural terms, but here it showed two different definitions.

1. vain: having or showing an excessively high opinion of one's appearance, abilities, or worth. OR
2. vain: producing no result; useless.

I think that the phrase "vain imaginations" can be thought of using both definitions. The people in the great and spacious building have these imaginations that they are beautiful, all-powerful, and all-important. Which, in a sense, is true in Heavenly Father's eyes. But, they do not see it in His way. They look for the glory of men. He loves them, but is sad when they behave in this way. Lehi did say that they mocked those who partook of the fruit of the tree (see 1 Nephi 8:27). This is what is sad. Jesus said once, too, if ye have done it unto one of the least of these my brethren you have done it unto me (see Matthew 25:40).

In the other sense, their imaginations produce no result in the long term. The things that they desire are trilling for a moment, but only end up in sadness. This is what Satan does to them when they listen to him, and will not support them at the last day (see Alma 30:60). Earlier, Nephi even said that large and spacious building did not have a foundation, or in other words, no support (see 1 Nephi 8:26) and great will be the fall of the large and spacious building (see 1 Nephi 11:36).

So now, let us not follow the ways of the world and enter into that large and spacious building, but help them through our examples to realize what they're doing and help them to come to the tree.