As to the quality of the rendered video, the only option I know of in Nero Vision is under "more" button on the content screen, "video options" at the bottom, "DVD Video" tab, and change the encoding mode from 1-pass to 2-pass VBR. This will result in some improvement (and take longer to render), but will not be too drastic of a change. The other factor is how much video you are importing, i.e. whether it has to compress it, but two 700 mb files should not cause any degradation for a DVD length disc.
As to playing them uninterrupted, when I burn several files using Nero Vision they will play uninterrupted as long as I select "When finished playing a title ... play next title" option under "more" on the select menu screen ... no need to stitch them together as one video file first.