Miguel,
you need to export your single streams as MPEG2-files. each one could have 8Mbit, you don´t need to divide the target-bitrate 8MBit by the numbers of your streams.
I do not know DVD Architect, but the best thing for authoring in general is to provide allready encoded assets to the authoring-application.
the most important thing for your multi-angle-feature besides bitrate is to use closed GOP and the same structure for all streams, otherwise you wont be able to create a multi-angle. if you could set sceen-detection within your encoding-programm – turn it OFF.
I don´t know if vegas could create/export valid streams for multi-angle DVDs, because of the special GOP-option which is needed.
otherwise you should export to AVI or MOV and use a 3rd party transcoding-application like Episode or Squeeze.
if your source is 23.98fps it will be transcoded to 29.98NDF NTSC with 3:2 pulldown, so to use 23.98fps is OK.
cheers
danny