Usually holes in this area are caused by the vinyl rubbing against the seat adjuster trim, and cracking/tearing
Get some grained vinyl sheet. Clean up the tear as best you can (removing the threads). Put in some stitches at the edges to prevent the good bits from further unravelling from the cloth trim.
Cut your scrap vinyl into a small square, and manouevre it under the torn material, and glue it in place with vinyl glue (Loctite etc). Its not clear from your picture if you have all the pieces of vinyl. If so, you could probably close the torn edges over the vinyl patch, and have a near invisible repair. If not, its going to be hard to have a totally invisible repair, but its in a discrete place, and will be no more visible that an entirely different panel of a different grain sewn in by a trimmer for £100.
Many years ago, Halfords (I know, spit) used to sell an interior trim repair kit, which, if memory serves me correctly, included a black filler material for repairing vinyl trim, which could improve the appearance of a repair. So, such a product does exist.
But, it might be worth seeing what a SMART repairer will charge. There are some pretty good examples of what a pro can do here:
http://www.smartfixrepairs.co.uk/smartrepairimages.html
These repairers probably use products similar to these:
http://leathermagicpro.com/Pages/vinrepprods.htm
The “liquid vinyl” can have a grain “moulded” into it to mimic the rest of the seat, but also remain flexible.
Personally, I’d try a DIY approach, with a flexible glue that won’t make it any worse.
PS. I might have some scrap black vinyl from a defunct 1996 Roadster, which I can post you if you PM me your address.