GT, the Great Vowel Shift occurred from roughly 12th-18th, with most of the major effects in place by Shakespeare. Two main things happened. First, we used to pronounce all letters. So the 'k' and 'gh' in Knight were pronounced. It therefore sounded like k-nee-cht (the ch here like the German ch, not our current ch). The 'e' in words ending in 'e' was pronounced. So all of the end words in Chaucer rhyme. (In fact, knight sometimes ended in e, and it was pronounced.)

The second, shift part of the process was a step down shift. So the long E became long I. In terms of sound, 'my' was pronounced 'me.' The long A became long E. So our 'meat' was pronounced 'mate-eh' (ending e pronounced). Short A became short E (bag to beg). In terms of sound,

short O (father)
becomes long A (fate)
which becomes long E (feet)
which becomes long I (kite)

AU (bought)
becomes long O (boat)
which becomes long U (boot)
which becomes OW (bout)

As you said about the evolution of the language, the Roman rule in Britain ended around 400 CE, the Viking influence around 800, with Alfred showing up 870s