Much depends on the point at which we decide that RP came into existence. William Gladstone, who was PM for four different terms in the late 19th century, went to Eton College yet his pronunciation gave away that he was from the North of England and his speech was criticised by RP speakers in the early 20th century.
Definition of vane. 1a : a movable device attached to an elevated object (as a spire) for showing the direction of the windb : one that is changeable or inconstant.
Vane, which used to mean flag, has a cognate in the German word for flag, Fahne, showing the original f. Vixen, finally, represents the southern pronunciation of a word that goes back to Old English fyxe, the feminine of fox. It was formed by a change in the root vowel of fox and the addition of a suffix -e or -en.