When Vegas 15 first came out, I had lots of problems with smartphone videos. I recently tried processing the same videos using a method learned on Vegas Forum a while ago. But just a few minutes ago, I saw the command "Add at Project Frame Rate" in a pop-up menu. That made everything I was doing manually now automatic. Either way, the results are better than expected. Playback is very smooth and audio is in sync. I use the same technique when I want to go from 29.97 constant fps to 23.976 fps for 3D Blu-ray movies.

When you right-click a video in the Project Media bin and then select "Add at Project Frame Rate," the video is put on the timeline at the project frame rate instead of the video's frame rate. The video and audio are both stretched. I tried rendering with smart resample disabled, but that wasn't as smooth as having it enabled. The trade-off for smoother playback is the length of finished video will be slightly shorter or longer than the original length. Give this a try if you have Vegas Pro 15 or Movie Studio 15. Make sure you have the latest versions.

Before I realized the "Add at Project Frame Rate" command existed, I was doing it this way in Movie Studio 15 Platinum:

(In this example the project frame rate is 29.97 fps and the video clip frame rate is 30.647 fps, variable)

1. Go to the Project Media tab and select the video clip you want to convert.
2. Write down the frame rate shown in the status bar at bottom of the Project Media panel. Our example will be 30.647 fps.
3. Divide the project frame rate, 29.97 fps, by the videos frame rate, 30.647 fps. The result is 0.978.
4. Add the video clip to the timeline.
5. Press "U" to ungroup the audio and video.
6. Select the video, right click it, and select Properties from the pop-up menu.
7. Enter the result from Step 3, 0.978, into the Playback rate field. Don't change the Under-sample rate.
8. Drag the end of the video clip out to its new length. If you drag too far, you will see a small notch appear at the top of the video clip. The end of the clip should snap to the notch. If instead the video clip's frame rate is lower than the project's frame rate, you will be shortening the clips. There will be a notch already showing.
9. Separately, time stretch the audio until it is the same length as the video. To time-stretch, you can hold down the CTRL key as you drag the end of the audio clip.
10. Select both audio and video, then press "G" to group them again.

This is a great tip, Robert! Thanks for sharing it!