Same problem here. When you record the audio and video separately, the track lengths are rarely the same.
My solution was quite simple :
1- When you start recording with the voice recorder, you should capture the red light coming up on camera, so that when editing, you know where the audio tracks begins.
2- Also capture the same red light coming off on camera, so that you know when the audio track should end.
3- Once you've set the beginning at the right place in the video, stretch or compress the audio track on any suitable editing software so that it meets the end point on the video.
Once that is done, calculate the percentage of stretch/compression that you had to apply to the audio track (if the editing software doesn't already show it, most do). Then you'll just have to apply that same percentage to every video you'll be editing in the future.
In my case, I've always had to set the audio track speed to 102% and it's always worked spot on.
Hope this helps.
