Was out of town in a conference, so i tested it today..
I can't see a problem here:
http://vbtube.com/vb4lite/ytm.php?do=watch&m=65
This is the first video in that category and next button is working fine. First video has no previous only next video.
And this is the last video in that category:
http://vbtube.com/vb4lite/ytm.php?do=watch&m=133
There is only previous button.
Where exactly is the problem you have ?
Can i have a link to reproduce ?