In Witcher series, Aldersberg is in south of Vengerberg. The book mentions, that Demawend and Meve took the battle at Aldersberg with Menno Coehoorn and lost. Altough, Aldersberg was still fighting, and Nilfgaards forces "were moving towards Vengerberg, kings Demawends capital".
The other issue - in Aldersberg, Meve says "So... now that we're alone... what's really going on?", Demawend replies "What, did you not here? I'm defending my capital". Okay, we can assume, that by speaking "capital" Demawend means Vengerberg, and as long as Aldersberg is south of Vengerberg, that's correct. Still, Vengerberg is not mentioned in the game at all (if I recall correctly). More to that, when you approach the city, you find stuffed unicorn, that belongs to Yennefer. And she was after all Yennefer from Vengerberg. :)