The concluding prayer of the prime office says:
℣. Dóminus nos benedícat, ✠ et ab omni malo deféndat, et ad vitam perdúcat ætérnam.
Why does the verb (perdúcat) split the noun (vitam) and the adjective (ætérnam) that describes it? Is this to emphasize ætérnam, by making it the final word? Why not say "ad vitam ætérnam perdúcat" (to match the construction of the preceding phrases)? Is this split noun-adjective construction common in Latin? Does it have a particular reasoning behind it?
The phenomenon at issue here is what is referred to as "hyperbaton": please click on this link for a definition of this term and for some varied examples from Ancient Greek and Latin. Note, by the way, that the very same example you'are interested in here has already been discussed in this previous post.
Here I'd like to add that this particular kind of hyperbaton exemplified by ad vitam perducat aeternam can also be found in classical prose authors such as Caesar and Cicero: note that the two following examples replicate the very same (!) type of construction. In these three examples the adjective (aeternam, iniquum, optatos) has strong focus. In my opinion, it is not the case that the verb turns out to be inserted between the noun and the adjective. I’d rather say that it is the adjective that is moved to the end and it’s there where it gets a focal reading. To put it in your words, this is to "emphasize aeternam, by making it the final word". As you note, the marked order ad vitam perducat aeternam can be compared with the unmarked/basic one ad vitam aeternam perducat.
ne forte in eodem loco subsistere hostis atque elicere nostros in locum conaretur iniquum (Caes. Gall. 8.16) 'In case the enemy tried to remain in the same place and draw our men into a disadvantageous situation.'
ad exitus pervehimur optatos (Cic. Off. 2.19). 'We are wafted over to the wished-for haven' (Perseus) / 'We reach the desired outcomes.'
In the linguistic literature on word order, Latin is sometimes referred to as a "discourse configurational language", in which pragmatic notions like topic and focus are encoded structurally rather than linearly. If you are interested in this issue, take a look at this answer, where you’ll find some hopefully useful references, from basic level ones to more advanced ones. Finally, I should add that, when dealing with this very complex issue, there is an alternative perspective on hyperbaton based on the role of prosody. For example, see this short paper. A more detailed version of this approach can also be found here.
