Maybe Germany was united in the Fallout world too, but it was West Germany that was annexed by East Germany, not vice versa?
I'm doubting the Berlin Wall ever fell in the Fallout Universe. As per West Germany being annexed by East Germany...Did the Soviets finally come crashing through the Fulda Gap and take West Germany?
Or are you referring to real world events...in which case the Federal Republic of Germany(West), and the Germany Democratic Republic(East), were united.
But I can't think of any instance, outside of fictional where the DDR annexed West Germany.
To quote/butcher "Buffalo Soldiers"
"Where's the Berlin Wall man?"
"Germany man, *censored* Germany"
"Are we in East or West Germany?"
"We are in East Germany!"
"No we are in West Germany you dumb *censored*"
"What's the difference?"
"*Censored* if I know..."