Demux your VCD's in tmpgenc's tools (or other demuxer, even DVDLab will do it, just say NO to Audio Transcode).
Convert audio to 48khz AC3 using Ffmpeggui (it's free).
Import the demuxed video and the AC3 audio into DVDLab.
The problem arises when authoring VCD onto DVD, because VCD audio is usually LPCM @ 44,100hz, and DVD spec is AC3 audio at 48khz. The transcoding from 44,100 to 48khz usually throws it off.