Recently watched Apollo 13 again. What is the white vapor that 1)Is cascading down the sides of the rocket, and 2)Comes out of the engine before ignition? Also, During liftoff, it looks like a bunch of white chunks or shingles are falling/flaking off the sides of the rocket. Any ideas?
My guess is that the chunks and cascading vapor are ice/water vapor from condensation on the outside of the rocket from the compressed fuel.
http://boards.straightdope.com/sdmb/showthread.php?threadid=26634
Check Here. You are basically correct in your assumptions.
Thanks for the link mavpace. Should I also assume that the white vapor coming out of the bottom of the rocket is water vapor(fog) from the LOX, or is that some type of engine “priming” or something completely different? I couldn’t find any reference to it at the spaceline site.
It’s “fog” from water vapor in the air condensing from the cold of the pre-ignition O[sub]2[/sub] spewage. The fog in question #1 in the OP is the same as the fog that rolls out of your freezer when you open the door–water vapor condensing out of the air because of the cold rocket body. It forms droplets on the sides (like on your glass of iced tea) which then freezes, forming sheets of ice which crack and fall off on launch.
note: Yipee! I can do sub[sub]scripts[/sub] 
You might also have noticed a giant billow of steam from the base of the rocket for the first few seconds after ignition. That’s from the cascade of seawater pumped over the base of the launch pad to keep the flame temperatures from destroying it (they DID want to use the thing again).