Vinitharya was the royal son of Valacar of Gondor and Vidumavi, daughter of the King of Rhovanion. When he returned to Gondor with his father, he was given the name Eldacar, a name he retained when he inherited Gondor's throne.
Winidas (in Old English) were the Slavs who dwelt inside Germanic territories. A possible analogy of the Winidas within the legendarium could be the Middle Men who belonged to Gondor, a suitable reference for the Northmen.
Note that it should be pronounced as Vinit-harya (as in plot-hole), and not as **Viniþarya.