Where did the numbers 460x345 come from? The proportion should be 1.363636... x 1. So you should be using 460x337 or 470x345. In fact, most compressed formats won't let you be quite that arbitrary; the sizes often have to be multiples of 8. So a close compromise would be 464x344 which would be a tiny bit taller than normal. But, whatever size you create, you can set a custom template directly in Vegas to that size rather than using 720x480. Click the Custom tab, then change the size to Custom and enter the values you want to use. Also make sure you set the aspect ratio to 1.00000 for multimedia files.

When you rendered to 720x480 that makes the assumption that the file will be played back with an aspect ratio of 0.90909.... but the player probably assumes it is 1.0 and squashes the image to make everyone short & fat. Choosing a frame size in the 1.363636...x1 proportion and using an aspect ration of 1.0 will guarantee unsquashed playback.