What Videopad version is being used?
Before the clips are placed on the timeline right-click in the preview window and change the Aspect Ratio to match the source clips - 9x16.
Then create text that fits within the image area. At export the text should then stay within that boundary, whatever the export resolution.
If this does not succeed, please share your project and someone will check it out. It's not difficult,
Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material you can link it in a personal message (PM) to me, via the envelope above. Be sure to FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT, and don't forget to share it.