I concur with the part about them being cheaper to replace than the whole
window.  If you live in British Columbia, or at least Vancouver, you should
keep a couple of spares.  They can be considered sacrificial there.  Or if
you lock them or don't have them, leave a slim-jim taped and labeled with
instructions for use so they don't break one of your other windows.  I just
left mine slightly cracked.
     Being a slow learner, however, it cost me 3 vent windows in 2 weeks. I
guess my learning curve approximates a straight line.

87 Westy SVX
86 Syncro
91 GL