You wouldn't be able to do 1500 slides without some work in DVD-A. If you use Vegas and output a video stream of slides, then you have to be careful how long the slide duration is, the encode bitrate, and the number of menus you create. 1500 slides at 5 seconds each is just over two hours of content, so you will need to adjust your bitrate accordingly. In addition, you can only have 99 chapters per title, so the viewer wouldn't be able to navigate to each slide individually (no skipping forward or backwards a slide at a time). If you try to use the slideshow in DVD-A, then you are limited to '99' slides in the sense that only 99 of them could have chapters (this is a dvd spec limit, not DVD-A). DVD-A will allow you to add the rest of the pics, but there wouldn't be any chapter points (not unlike if you used Vegas). The problem with DVD-A is that it creates a video stream for the slides, rather than a single "I" frame per slide (with the ifo file telling the set top player how long the slide should display). The importance of this is that 1500 slides encoded as a stream results in several gigabites of data, whereas slideshows encoded as "I" frames only result in only several mb's. Most other authoring apps do it this way, but not DVD-A. However, one workaround is to create a 'music' compilation and use the slides. It appears as though DVD-A creates "I" frames only when it does this. You could have a single 5 sec blank audio stream that would be associated with each pic.

Jim