See "Rendering projects for use in DVD Architect Studio" in online Help.
If you’re using the MainConcept MPEG-2 encoder in Movie Studio software, use the DVD Architect PAL video stream or

If you render to DVD architect you have to do the audio separately. If you use the PAL program stream that would do the video/audio in one step. You can decide if you like the widescreen format option better than keeping the original format, try them both and see which you prefer.

As often reminded here, if you don't do the audio separately, DVD A will recompress the audio.
Vegas rendering settings for a DVD
Avoiding recompression in DVD Architect

I have a a Vegas project file rendered as a single mpg (incl audio)
Using MainConcept DVD PAL template (make DVD wizard points to this)

I have audio included, and VBR set for 2 pass, rest is default.

I then open DVD arch and use the make DVD option (Single movie) selecting AC3 as audio.

I have no problems dealing with Audio & Video separately, but thought Movie Studio & DVD architect were integrated enough to avoid this step.

If there are any guides as to step to do this neatly - glad to know of them ?

P.S. ... can you set options on this forum to get auto notification of replies ?

>>>If there are any guides as to step to do this neatly - glad to know of them ?<<<
The "Vegas rendering settings for a DVD" link outlines the steps rather well.
Replace NTSC with PAL.

>>> can you set options on this forum to get auto notification of replies ?<<<
No.

"I have no problems dealing with Audio & Video separately, but thought Movie Studio & DVD architect were integrated enough to avoid this step.
It's not a Vegas / Architect integration thing.
It is simply better to provide Architect with elementary, compliant streams for muxing.
Not doing so will always cause the audio, video, or both to re-render, costing you both time and quality.

Although, it would really be nice if Vegas had some sort of function that allowed the user to set up both audio & video renders and run them both in one step.

An inexpensive plug-in you could try from "Partner Products".

Should work in MSP 12&13, VPro 9 to12.

"I have no problems dealing with Audio & Video separately, but thought Movie Studio & DVD architect were integrated enough to avoid this step."

I wasn't very clear in my other post, if you use the rendering template "PAL program stream" it produces a .mpg file that can be imported into DVD Architect with both video and audio in one step. Either template will work with DVD architect, you don't have to choose the DVD architect template to be able to import the rendered file into DVD architect to create the DVD.

I haven't done it in a while, but I'm pretty sure when I rendered SD video to the NTSC program stream template, burned a SD DVD, and played it on my 1080p TV that the player stretches it to fill the wide screen format anyway. That's why I said try both formats (regular program stream and widescreen program stream) for yourself and see what happens.