Chris, I replaced the glass in my 56 but not the vent windows... figured I leave the cracks and imperfections for character. Anyways, if we presume that the vent window metal frame is similar to the window, I pulled the broken window pieces out of the frame... had to use a narrow screwdriver in some places to pry the glass out (wear eye protection!)... Once out and I scraped any remaining rubber in the channel then cleaned, painted, etc the metal parts. I soaked the new rubber piece in motor oil overnight... the next day I layed the rubber in the channel and worked the glass in place. The oil makes the rubber break down a tad and then when dry seals up nicely. This trick was passed along to me by one of the techs at Checvy Duty. I'm thinking you can use the same techniques for your vent windows.
