Poor quality when transcoding from DVD audio to MP4

[cjfred]

I'm trying to use VLC to convert a spoken-word audio stream from a DVD to MP4/AAC audio (m4a) format, using two channels at 64 kbps. However, when I do that, the audio comes out very "tinny", with a significant level of distortion, seemingly due to compression artifacts, in the mid-to-high frequency ranges. When I convert to MP3 instead, then use iTunes to convert to MP4/AAC, everything is fine and the audio sounds great. Because the re-conversion takes time and theoretically results in loss of quality, I'd prefer to have VLC write directly to MP4 audio format. I don't want the files at a higher bit rate, because 64 kbps is quite sufficient to represent spoken word audio. Is there any way to make VLC encode better quality MP4 audio, without the high-frequency artifacts, as it does with MP3 audio?

It should be noted that in the process that currently produces acceptable audio, I'm having VLC write out 256 kbps MP3 rather then 64 kbps MP3, and allowing iTunes to transcode it to 64 kbps MP4, in an attempt to minimize the quality loss associated with transcoding.