In the past these rubber dog bones came in two lengths, a short (front) and a long (rear). If memory serves me right, I think the shorts were 1.5" long and the others were 2.5" to 3.0" in length. I would also guess that the Vibe would use the shorter ones on the rear but you need to measure that for yourself. Also check that you have room between the spring and the shock for these to fit.
Putting these spacers in will raise the back of the car but will also stiffen the springs. The easy way to think about a coil spring as that it is nothing more then a torsion bar wrapped into a coil. Like any torsion bar, the shorter the bar is the harder it is to twist. By putting these spacers in, you are taking out a coil of movement from the spring or shorting the torsion bar. This in turn makes the spring stiffer which will hold the car up under load and will make the car stiffer at the rear when unloaded.
The link was an example but here is another link:
https://www.amazon.com/Superior-18-1601 ... ng+spacers
I do not recommend the metal spring spacers because they have no give to them and could damage the spring.