"What Were They Like" begins with a series of questions about "the people of Viet Nam." The nature of the questions, along with their formality, imply that the poem's first speaker is likely a historian or an anthropologist, someone who is trying to understand a culture that no longer exists. This, combined with the title, provides some context for the poem. In the world of the poem, the Vietnamese people and their culture have been wiped out, and now someone, most likely a historian, is trying to understand what they were like.
In the first question, the enjambment after "Did the people of Viet Nam" draws attention to what's missing in the world of the poem: the Vietnamese people themselves. Note that the movement of the question across the line break is from general ("Did the people of Viet Nam") to specific ("use lanterns of stone?"). There is a sense of the speaker trying to understand an entire people based on a single detail. It is implied that knowing whether the Vietnamese people used stone lanterns will tell this person something important about their way of life.
The syntax (the arrangement of words) in the second line places an emphasis on the word "stone" because it falls at the end of the line (a more casual syntax might have been "use stone lanterns," in which case the emphasis would have fallen on the word "lantern"). By emphasizing the word "stone," Levertov allows for certain connotations to enter the poem: one might think of the "Stone Age" and the notion of "primitive" peoples and tools.
The second question is structured similarly; it again moves from general to specific across the line break—"Did they hold ceremonies" being general, "to reverence the opening of buds" being specific. "Buds" refers to the early form of flowers. Basically, the speaker is asking if the Vietnamese people had some sort of cultural or religious ceremonies to mark the start of spring.
The questions are seeking information, but there is also a sense of othering already happening in these opening lines. The question of "what were they like" followed by these particular questions—did they use stone lanterns, did they celebrate "the opening of buds"—implies another question, one that is more telling: how were they different from the people represented by the speaker?