Well...
You say it's fine now, does that mean the jerkiness went away? It could have been caused by wrong ivtc settings.

Do you want to be resizing? You are using LanczosResize(640,352) (making the frame smaller). If you are first making it smaller and then resizing back to dvd resolution in vegas that could of course cause some loss of detail. Comment out the line if you don't want that.

I feel I don't really have enough information to help you properly - you also mention aspect ratio; you don't need to correct the aspect ratio if you're making a dvd for example, your project should simply be in 720x480 for ntsc.