I was having some trouble with a drip in my softtop from the seam over the back seat. Water was leaking into the folded up portion somewhere and then dripping out the end of that fold and putting a puddle on my daughter's carseat.
I went to Gander Mountain yesterday and got a tube of tent seam sealer from their camping section that said it was suitable to use with vinyl tents (some said nylon only). I followed the directions on the tube and cleaned the seams with cotton balls and alcohol, let it dry, then applied the sealer to the seam.
Overall I think it turned out well, the sealer is a little bit shiny, but not as much as it looks in the picture (glare from the flash). I think that the shininess will fade once it sees some sun and dust. I took my jeep to the carwash today and had no leaks when I left.