The short answer is that we don’t know. “Imphe” and “Bire” (or “Ebire,” in some readings) are not known from other extant sources. This probably means that they were either invented by the composer(s) of the Chanson de Roland, or that they represent spellings of extant names distorted beyond immediate recognition.

This doesn’t mean that people haven’t tried! Miroslav J. Hanak considers precisely your question in a 1971 article in Romania, entitled “Sven Forkbeard, Bjoern Ironside and the City of Iomsborg in the Chanson de Roland," which you can read here. Hanak refers to heated nineteenth century controversy over the identification of these toponyms, surveys extant theories (ranging from Portugal to the Euphrates) and ultimately proposes his own. He suggests that “Imphe” is “Impne,” which he identifies as Wolin in modern Poland (a leading candidate for the site of the semi-legendary fortress of Jómsborg), and that “terre de Bire” refers to “the land of Bjorn Ironside,” who appears in a single Latin chronicle as “Bier costae ferrara.” King Vivien is Sveinn Forkbeard (d. 1014) or perhaps his grandson Sveinn Ulfsson (d. 1076), with his name changed to appeal to French sensibilities.

Is this convincing? Err… I’ll leave that to your judgment. Suffice to say that this etymological hoop-jumping (not to mention the plethora of other solutions that Hanak reviews!) is an indication of the obscurity of these place names. That may be the point in the text itself. Having returned to his capital of Aix, grief-stricken and exhausted from battle in Spain, Charlemagne receives a summons to yet another far-off conflict; one which he, as well as modern commentators, might have difficulty even plotting on the map.