If you are authoring in DVDSP then it is a waste of time to go through the Share process. You can use Send to Compressor, or make a ref movie and just drop into Compressor then take the m2v and ac3 files into DVDSP and author.
I’m not a fan of VBR encoding when you can use high bitrates. I would always you a CBR encoding anytime the average bitrate is over 6Mbps, which it is if you are using 90 minute preset.