The word vale, adds a scholarly feel to the poem, and allows the poem to flow better overall. It is also a huge difference in the rhythm, as it has only one syllable rather than two.
The word margin was used to add a new taste of vocabulary. Margin's other meaning is "an amount by which a thing is won or falls short" so in using this word, one has the feeling that it is almost but not quite the analogy used. So it's an implication to the poem's analogy itself, giving a sort of fourth wall experience.
