Since BILSTEIN does not make struts or shocks for a Vibe, your not going to be putting those on. Of coarse you can do anything if you throw enough money and time at it. To make a set of BILSTEIN work on the Vibe, you would spend more than the car is worth.
Shock companies have different models of shocks to fit cars. Not all Monroe shocks are the same and not all KYB shocks are the same. On general reading and self experience, I would say the KYB shock is as firm or slightly firmer than the OEM shock. The Monroe shock is valved slightly softer then a new OEM shock/strut. I myself think that the Vibe rides to stiff with the 17" rims and will be putting on Monroe struts/shocks on when the time comes to change them.
The other thing to consider is that your car is ~9 years old. The struts/shocks have degraded over time and mileage. You can be sure that replacing them with any new shock is going to stiffen the car up from where it is at now.
SOME additional info. Im a bit spooked with the driving speeds on the high ways... 80 is the new 70 (speed limit)
If you in the slow lane you better be doing 80... if you pass yur doing 85-90... So I want better that oem
Shocks and struts...
Shocks/struts are not going to help with this problem.