These are not the same word in two different forms, although students sometimes believe so. They have different origins.
via
Lewis and Short define this as ‘a way, in the most general sense.’ Its meanings include highway, path, road, street, journey, march, route, passage, channel, lane, duct, track, method, mode, manner, fashion. It is a word that connotes movement, being from the same root as the verb veho, the English vehicle and wagon, and the Greek ϝόχος and ϝόχημα.
vīcus
The long letter i reveals that this is a word of different origin. It is cognate with the Greek ϝοίκος, and it means collectively a row of houses in town or village. In the city it can mean a quarter, or a street of houses. It does not contain the notion of movement; and later it comes to be used for a whole village, or a country estate. From this origin comes also the English -wick or -wich, to be found in place names, and it may be noted that even as late as Shakespeare’s time, the common noun wick was sometimes used to mean a town, village or hamlet.
English place names include Chiswick, Berwick, Shapwick, Droitwich, Greenwich, Harwich, Norwich, Sandwich, and Ipswich—this last being called Gipeswic in the 10th century. Sometimes the name reveals the origin of the settlement as being a group of houses that were built around some place of special importance, like Northwich in Cheshire, around the salt mine.
It does seem useful that the student of Latin should notice how important is the quantity of vowels. To the speaker of the language in classical times, and for the speakers of the Romance languages ever since, the long or short pronunciation of a vowel has been carefully observed, and as a consequence, so has the stress on syllables, which in the modern languages still corresponds almost unerringly with the original Latin.
No Roman of Classical times, and indeed no Italian now, would imagine that vīcus and via were variants on the same word.