Just had this happen to me over the weekend. Unfortunately, I couldn't find the plastic pieces once they broke off since I was driving. I couldn't get the Vista Windows to close completely. What I did to get it to close was:
1. I used 100% Spray Silicone on all the slides including the rear groves.
2. Once this is done the Vista Windows will slide back and forth easily by hand.
3. I was able to close the Vista WIndow myself by using the Initialization process listed above. while keeping pressure (towards the front of the vehicle) using a wood block against the metal frame on the bottom front of the window (it has a 1/8" lip)
As stated above I can use the vent feature and the shade screen still functions.
The Scary parts was I always kept the rails clean and lubricates using 100% silicone, and when the plastic clips broke I was on the freeway with the Vista Screen half open, when the plastic clips broke the Vista Screen slammed back as far as it could go, for a second I thought the entire glass had broken and I was in for a world of hurt! If the Glass would of come dislodged or broke apart, It could have caused a major accident.
I did file a complaint with NHTSA this morning.