Unable to replicate your result here. What version of Videopad is being used. Here is the latest. For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.
Overlaid and positioned text onto a 31KB image and exported at 900 x 506, leaving the Letterbox A/R choice unchanged.
The text position on the Mp4 result (right) was identical to the preview (left).

Upload and share your image and someone will double-check it. Or share the PORTABLE project, so an export can be tested.
Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/