Hey Paco–
Here’s my stab at your questions:
(1) You don’t have to convert anything; Vegas can do that on the fly.
(2) I would transfer the footage to your internal drive, yes.
(3) I think I would match the project settings to your iPhone files. It may not matter, but Vegas may do a better job throwing away frames from the higher frame rate than interpolating new frames from the lower one.
(4) I don’t think you have to adjust source settings.
(5) I’m not sure what “bouncing” a file is, but don’t over-think things; Vegas is very powerful, and you should be able to add all your effects, transitions and graphics in one big mixing bowl. In a realtime preview it’s gonna look clunky because it’s got a lot to think about all of a sudden, but render it and all’s good.
(6) You might search the forum for DVD info, because it’s all there. You’re probably looking at MPEG2 and (I forget) for audio, because I haven’t had to burn a DVD in a couple of years.
–Kelly