De or het vonnisbrief?

The correct Dutch article is:

de vonnisbrief

How to use vonnisbrief

the vonnisbriefde vonnisbrief
this vonnisbriefdeze vonnisbrief
that vonnisbriefdie vonnisbrief
our vonnisbriefonze vonnisbrief
your vonnisbriefjouw vonnisbrief
every vonnisbriefelke vonnisbrief
which vonnisbrief?welke vonnisbrief?
a big vonnisbrief / the big vonnisbriefeen grote vonnisbrief, de grote vonnisbrief