"Proso / estos versos"

"I prose / these verses"

It seems to me that that is the only possible translation. You can't say "I write these lines," or "I put down these verses," or "I prose these lines." You have to preserve that pun and that antithesis (prose/verse). That's the point of the phrase.

Vallejo invents a verb that doesn't exist in Spanish: "prosar." So we do the same in English. He could have said "prosifico" (prosify) but that would have meant something different: to rewrite a text in prose (like "versificar").

What does it mean to prose verses? To write prosaically in verse? If we look at the poem where this phrase occurs, it is obviously a sonnet written in endecasílabos. There is one peculiarity: assonant rhyme (instead of consonant) is highly unusual in the sonnet form. Less prominent rhyme, enjambment, and a colloquial tone combine to make the poem prosaic. There's also a certain modesty in the authorial persona--combined with a self-dramatization that's not exactly modest: "César Vallejo ha muerto." "I'm not a real poet, I can only prose together this sonnet."

There is the poet César Vallejo, author of these lines, and the character César Vallejo who appears there. There is a division in the subject position that might be interesting to study, but I have to go teach something completely unrelated in 5 minutes!