Well depends on how well you want it fixed,if it's white vinyl you can use a bit of white silicone, that will help keep it from getting bigger,just put some into where the tear is and smooth some on the outside, now to get a smooth finish put some water into a small cup and some dish soap and wet your finger with that and that will give you a smooth finish,now this only a temp repair or you can get a mobile vinyl repair truck to come do the job and they do it on site prices vary , and some of these guys do a great job good luck.
Our experience is that vynil repairs are hard to effect. You can sew/ stitch the tear to prevent it from getting larger and to make it look a bit better. Or you can use sail repair tape ( comes in colors and is waterproof). But both re obviuous fixes that still leave you with a cosmetic problem.
The "real" fix is to have a canvas/ upholstery pro recover the seat. Depending upon the seat, it may behoove you to just buy a new one.
Just had mine repaired by a vinyl or leather repair guy. You couldn't even tell it was ever there. An very inexpensive. I think he charged around $50 to fix two little tears, and you can't even see where they were.