the only way to know whether adding a feature (like drop-shadow) will ruin your app is to try it.
there's probably no instance where a vector background image will perform as well a bitmap. but, your bitmap's memory requirements can't exceed the available memory and, to call something a background image means, the background image is always in the background and its bitmap representation doesn't change frequently.
because it's so much easier to use vectors with flash, you would usually create a mostly vectorized app (using the cacheAsBitmap and cacheAsBitmapMatrix everywhere it's helpful but being mindful of the limited memory you can use) and test performance frequently during development. if performance is adequate, you're good.
if performance is unacceptable, you'll need to check if you're exceeding the available memory and losing the benefit of cacheAsBitmap or if you're putting to much strain on the processor using vector manipulations.
Thank you so much kglad - that's very valuable information.
Do you think these same general rules apply when exporting flash content for use in iOS and Android apps? And that I should assume vector as an end product makes the most sense on PC applications? Thanks again.
yes, they apply to apps made for mobiles and desktops/laptops.
the biggest difference between creating apps for mobiles and non-mobiles is you're much more likely to need to optimize when creating mobile apps because mobiles have poor performance and limited memory compared to desktops/laptops.