Word of warning - Any recumbent is designed for a certain range of reclines. A Vision has a short wheelbase; in fact older ones were *very* short. Lay the seat back too far and it has the effect of putting the rider's weight high and over the rear wheel, which is bad for stability. The problem is, hardshells are designed for use in a *very* reclined position. They don't have much butt area; they're designed to lie on, not sit in. I would not expect a hardshell to work well on a Vision.
Thanks for the info. I have searched (& found) some hardshell seats and even if I did want to retro-fit my bike, the seats cost more than I paid for the bike.
I don't really know how old the bike is but I'd guess about 10 - 15 years. The wheelbase is pretty short (I haven't measured it). Anyway, just interested an I understand the center of gravity thing. I'll probably just leave well enough alone. Maybe look for a newer bike rather than just upgrading.