Converting to AAC in Foobar with the Fraunhofer/Winamp encoder:

1. download the latest fhgaacenc (https://github.com/tmkk/fhgaacenc/downloads)
2. download the latest Winamp installer and extract it (with 7-zip or similar)
3. from Winamp, copy enc_fhgaac.dll, libmp4v2.dll and nsutil.dll to the same folder as the fhgaacenc files
4. in Foobar, setup a new encoder with fhgaacenc.exe, the extension m4a and desired parameters (for example: --vbr 4 --ignorelength - %d)

notes:
Instead of step 2-3 you can also just install Winamp.
See the included readme.txt from fhgaacenc for more info.