FinallyBoating

You don't. Leave it be. Unless the upholstery is real thin, you'll never get the fasteners tight enough to prevent the grabrail from rocking a bit when loaded. (OK,..you might...but what stops that hole you poked in the vinyl from tearing and geting bigger over time?)

Can you remove just a small section of the upholstery to install the rail? And would you be happy with that look? If so, side panel cushions are often bolted on--if you reach behind you will feel the nuts. The studs are usually installed in the backer before its upholstered. I am assuming this is cushion installed flat against boat structure, not a trim piece that is only fastened along its top edge and unsupported along its bottom edge. If the latter is the case, its ubsuitably sturdy for a GR install any-hoo.

Now, you might be able to get someone--a welder, machine shop...to make you up a J-shaped rail. If they made the short leg of the J flat, you could remove the cushion, bolt the flat plate of the J on to the boat, then re-install the cushion. have them make the J with enough "gap" to account for the cushion thickness.

Why do you need the grabrail? is it to lend support to a passenger in a seat? If that's the case consider adding armrests to the seat, or swapping the seat for one with armrests. Or putting a footrest on the seat base and/or forward of the seat base. A person can secure themselves fairly well with a footrest.

I wish I had an easy answer for you.

G'Luck

Kevin

Now, if you'll take the rest of this response from the perspective of backyard engineering, perhaps I can float some ideas that will help you to decide whether to proceed or not.
How thick is the cushion? If its thin, like maybe an inch or so, you could drill through it and bolt on your rail and be able to sock down on the nuts enough to make a secure fastening. Maybe.

But, what's to stop that hole in the vinyl from getting bigger over time? In

So, we have to make a grommet effect of some sort without turning you into a tailor...hmmm..how we gonna do that?