Yes, you are correct that the second problem did not directly involve Sony Vegas (or DVD Architect). The disc image input to the programme related to a file that had been produced in DVD Architect, however, so if there was a shortcoming in this, then it could presumably be passed on to the disc image.
I must emphasise that I have successfully burned DVD-R files on the unit (employing ULEAD Movie Factory). This is what makes me think the problem is probably software, rather than hardware, based.