This is caused by the TV overscan. The TV zooms into the picture and this will cause some of the picture to be outside the visual area on the TV screen. The reason for this is that TV screens are not perfect in aligning the picture and by showing a smaller part of the picture there will be no picture edges visible.
This is a normal behaviour of the TV. What you can do is resizing the picture to a smaller resolution and add black borders around it (like letterboxing). I normally do this with avisynth and create the script with FitCD.
Alternatively you can do this within TMPGenc by using clip frame and arrange setting center custom size.
Check this thread:
http://www.dvdrhelp.com/forum/viewtopic.php?t=171661
But you can also just place your subtitles with more margins to the bottom and the sides of the picture when you encode to VCD.