DVD does not support 1080. Regardless, my suggestion is to first do your project in 1080. The project settings suggested by Brian are fine, although my preference is to set the frame rate at 60P rather than 60i. Rather than burning to a disc, consider using a media player to deliver your production. Many newer TV’s already have that capability built-in. If you need to or want to make a DVD simply using the appropriate render template within Vegas. Be aware that the quality of your production will take a big hit. I wouldn’t recommend this option.
By the way, what slide scanner are you using? My suggestion here is to scan at max resolution. Hopefully, your scanner has some way of automatic dust and scratch removal. Since this is probably a family archive, do it in the best way possible. I re-scanned my family collection (12000+) some years ago using a Nikon LS-5000 at 4000 dpi. It’s a very tedious and time-consuming process. With 1400 slides, having it done at $.25 per slide may be a bargain. Good luck.
wwaag