well there is an inventory limit of 100 items (250 for gold accounts) so if you don't want it to fill up you will have to put at least some items in your vault.

keeping money in the bank can help prevent some accidental unwanted purchases. also i personally like the feeling of tucking it away nice and safe, watching the number go up every time i make a deposit.

Okay good point. Thank you. But I don't need to use the vault to keep my items safe, right?
How many things can I store in the vault? Also I have an Ooh La Layers in there...with the new wardrobe update, is that useless now?

No, you don't need to use your vault if you don't want to. There's currently no limit to how many items it can hold though.

Ooh la layers has been pretty obsolete for awhile now (I think they still worked for adding layers in the v3 wardrobe? But it would have been far more expensive than just buying them in the wardrobe itself). It's pretty much only useful as a book now

About things affecting items in the inventory: The Red Rreign can burn your items to crisps if you give him an item with "wreath" in its name. The Rreign is only available in November though, so it's still a while untill then.