The pattern of the articles the and a/an being affected by the sound of the following word dates to Old English. The use of the stressed form as an emphatic does as well. It probably goes back further. There are similar traits in the other Germanic languages. German routinely reduces its definite article die ("dee"), to approximately "duh", and its indefinite article ein ("ayn") to approximately "uhn", as well. Except when emphasized. We even have traces of emphatic "a" in English which is now completely archaic -- except still irresistible in "an historic moment". That probably counts as a fixed expression now. But etymologically speaking, when someone says that they're saying "one historic moment" and emphasizing its distinctiveness. Germans would say "ein historischer Moment" and it's a safe bet they'd say ein (one) and not the usual "uhn" (a).

In BrE, "the" before a vowel is pronounced "thee". So I'm not sure what you're saying.