Anybody have any tips for flattening out wrinkles and waviness in Vader and Obi-wan vinyl capes? I have a couple that are really a mess and wish they were straighter.

There are a number of ways but one of the safest is to dip the cape in hot (not boiling) water for a few seconds to soften it. It should then be quickly dried and placed inside a heavy book, you can put extra weight on top of this if you like. Leave it for a while and this should remove some of the major creases but it'll never be 100% flat like an unused cape. If the cape is brittle it's best just to leave it as you may end up damaging it further.

I've used an office laminating machine a couple of times, which produced superb results, placing the cape between two sheets of heavyweight paper and set on low temperature.

I use the hot water from my Water Cooler (hot , but drinkable) and set the capes in the glass for 15-20 seconds. Then I lay them out on a flat surface with a book on top. Works every time

I use my lightsaber to heat up the cape and then flatten it with a stampede of gerbils

Seriously though, I've often used a hair dryer on max heat until it becomes softer and more pliable (20 seconds or so), then use a heavy book on the kitchen surface (make sure whatever you press it with doesn't have a pattern or you'll be doing it again! )

Flat clean surface. Relatively heavy weight on flat piece of plastic. Wait a few days, should be wrinkle free.