This is such a large and major theme in Tolkien's works that we probably have no one discussion of it. However it does come into most of our threads at some time or another.
From their language and culture (apart from their extensive use of horses) the Rohirrim are more closely based on the Anglo-Saxons than the Vikings, although Norse myth did enter into Tolkien's works in other ways (many of his names are derived from Norse texts, and the Norse legend of Sigurd and Fafnir inspired his interest in dragons).
Tolkien drew themes, names and inspiration from a vast wealth of sources, most of them medieval in some way or another. The name Eärendil is derived from a line from the Old-English poem
Crist: "éalá éarendel engla beorhtast ofer middeangeard monnum sended" (very roughly "Hail Éarendel, brightest of angels, over Middle-earth sent to men." and indeed 'Middle-earth' is the meaning of the Norse
Midgard and Anglo-Saxon
Middeangeard. The phrase 'third time pays for all' is taken from the Middle-English poem
Sir Gawain and the Green Knight and there are a huge number of other links, allusions and borrowings from almost all of the medieval European world. Keep browsing and you'll find some very interesting parallels, both those drawn by Tolkien intentionally and those drawn by others, which are often less believable.
You can find good searchable on-line English dictionaries
here and
here. We'd all rather wait a little longer for a post to appear while its author looks up words they're not sure about than see the dreaded 'sp?' make an appearance.