Well I tried again a little differently and it worked. First I used vision to merge the videos and not allowing it to reduce the quality to fit. Having one large mpg, i used tmpenc to convert to vob and then dvdshrink to shrink and burn. This time it worked.
Before I think my merged file was reduced to fit and then converted to vob and hence didn't need shrinking when burned to dvd.
Who knows maybe it was a bad burn too? Really i have no idea
But i used my sony dw-g120a and verbatium 16x media and burned at 16x.
Nero shows no errors...