If you know what manufacturer your windows are from and want to reach out to them to get the exact spline they used, that is always best. The .200 round spline has worked for me in all sorts of different panels so it would most likely work for yours. It doesn't have to be an exact replacement, just needs to securely hold the vinyl in place. Maybe try a sample of round before getting too far into it.
The spline in my window is flat. Is there any problem with replacing it with the round spline you recommend?
If you know what manufacturer your windows are from and want to reach out to them to get the exact spline they used, that is always best. The .200 round spline has worked for me in all sorts of different panels so it would most likely work for yours. It doesn't have to be an exact replacement, just needs to securely hold the vinyl in place. Maybe try a sample of round before getting too far into it.