Anna Marina:
The Bible does not say that Jerusalem fell in the 18th year of Nebuchadnezzar anywhere. But it does say it was under seige.
You've quoted Jeremiah 52:29 after that statement. However that verse, referring to Nebuchadnezzar's 18th year excluding his accession year, does refer to exiles taken after the destruction of Jerusalem (587 BCE) rather than the ongoing siege. (Also note that the reference to the '7th year' excluding accession year at Jeremiah 52:28 is the same year as the '8th year' including accession year at 2 Kings 24:12.)
On the other hand, the reference at Jeremiah 32:1 includes the accession year as Nebuchadnezzar's first regnal year, and does indeed refer to the ongoing siege the year (588 BCE) prior to the city's fall.