Actually, I would replace the glass while it is out of the vent window assembly. With new glass and new butyl tape my experience is that it is a VERY tight fit and takes lots of careful finesse and pressure to seat it in the frame; more pressure than you'd like to place on the assembled frame while it's back on the truck.
I have found that gentle heating up of the tape really helps it
"squish" while the glass is being seated. I use a heat gun or the handy pizza oven in my shop!
