Hopefully you have gotten the pins out by now, if not, do as the other poster said in previous posts and tap the pins out with a flat end punch. You will have to buy new pins. You can buy them from a local bike shop or From Fabio at the Vespa Super Shop. If you buy them at a local bike shop you may have to file them down a little so they fit properly. There are slightly different widths in these pins so check it against on e of your pedal cranks.
On to why your pedal wobbles and the chain won't stay on.
You will notice the Grande has a "subframe" that the pedal crank goes through. The rear wheel and engine also attach to this Subframe.
What has happened is, someone has been riding with the left and right pedal in an UP and DOWN position and the DOWN pedal has hit the ground. When this happens the subframe is usually damaged and the round hole where the pedal shaft goes through the subframe belongs elongated or tear dropped shape. The pedal shaft no longer fits securely through the holes and thus the wobble occurs and the chain comes off.
How to fix.
Remove the left pedal crank arm. Squirt some WD40 around both sides of the pedal crankshaft where it goes through subframe. Now tap the shaft through the subframe noting where all pieces are so you will know how to reassemble.
When you get the pedal shaft out you will see two black plastic grommets (one on each side of the subframe) that fit in the frame that holds the shaft in place. One probably will come off with the pedal shaft.
Inspect the right side of the subframe that is usually the one that is damaged or torn. It is very difficult to bend it back out because that area of the subframe is boxed in. There are two ways to repair the damage, one is a quick and temporary way and the other is a permanent way.
The quick way is to cut a piece of metal that is about the same thickness or gauge as the subframe metal. Cut a piece that is about 2-3 inches wide by about 6 inches long. Use a pair of pliers to shape this metal into a cone shape (like an ice cream cone shape). Stick the small end of the cone in the damaged subframe hole and tap it in and leave about a 1/8 to 1/4 of an inch lip so the cone doesn