Beowulf survives in a manuscript that was probably written in the early 11th century, and there's a lot of debate about when exactly the poem itself was originally written; you see dates between the 7th century and 11th, but it was probably the 9th or 10th. This means it was almost certainly not one of the first vernacular (English) poems to be written down; that honor might go to the Dream of the Rood, which survives on the Rutwell Cross from the 8th century, or Caedmon's Hymn (which survives in Bede, in Latin, from the 730s, but was pretty indisputably originally written in English). And there's the Y Gododdin, which was probably equally early if not a bit older, written in Welsh.

Of course, we're talking about writing in the vernacular; we know that there was a vibrant culture of storytelling across England (and, honestly, in every culture), and that was definitely happening in the vernacular. The large number of Old English poems and histories that survive from the Anglo-Saxon period aren't remarkable for being in the languages that people spoke; they're remarkable for being preserved in those languages. And that is a somewhat unique thing we see in England during the early middle ages. All the laws were written in Old English instead of Latin (Frankish and Visigothic laws were in Latin), and things written in Latin like Bede's history, many saints' lives, and much of the bible, were translated into Old English, copied, and preserved.

We're also at a tricky point for the idea of the 'vernacular' language. Many parts of the post Roman world, like France, were speaking languages that had started out as Latin and were still in the processes of evolving toward their modern descendants. When Gregory of Tours wrote his history in 6th century France, he wrote it in an educated Latin that's certainly much more sophisticated and traditional than the language the peasants were speaking, but I don't know if you could say it wasn't the vernacular. It was just very old fashioned compared to the contemporary, (d)evolved Latin that was well on its way toward becoming French. And many churchmen - at least, the ones well enough educated to write - had a grasp of the basics of classical latin (though often little more than the basics). So in those regions, the contrast between Latin and the common language was a little more like our relatioinship to Chaucer (we can recognize that middle english and modern english are related, but we can't start talking in middle english without proper training). It wasn't, perhaps, as unnatural for people in France and Visigothic Spain to conduct official business in the older, more traditional, more religiously meaningful language as it was for people to use Latin in England were, as far as we can tell, the Roman language had been largely replaced on a popular and official level by languages more closely related to Germanic dialects.

At all events, if you want to know more about translation from Latin to Old English, this book might be a helpful starting place.