SimVibe Chassis Mode and SimVibe Extensions Mode behave quite differently.
Chassis mode assumes that you have transducers at all four corners of your simulator and as much as possible, it does not make assumptions about the construction or rigidity of the simulator. NOTE: You can use SimVibe Chassis mode with only 2 transducers. It will just collapse the front/left and rear left into Left and the front/right/rear right into right.
When you hit a bump or rumble strip with the front-right for example, it will primarily output that to the front-right, but to a lesser extent, will also output it the other front-left and rear transducers just a short time later as would happen in a real vehicle. No vibration is limited to only one area of the vehicle. With this method, a rig with no cross member between the front-left and front-right will still feel the minor vibration resulting from a major vibration on the opposite side of the chassis. Even when the rig does have a physical cross member allowing the vibration to transfer, there is no loss of realism.
In SimVibe extensions mode, there is the ability to localize an effect to a specific transducer but these are meant for placing transducers on things like the Seat, Wheel, Shifter, Pedals, etc.. I don't think this would be a solution for you if I understand your request properly.