If you read 'The hostile hospital' (book 8, I believe) there is a part where Count Olaf has Violet and is going to perform a 'craneoectomy' on her. so, when she is about to be 'operated on' (just before Sunny and Klaus come and rescue her) she is wearing a hospital gown.

but, then, the hospital catches on fire so the 3 kids run for it and Violet would have no time to get her clothes.

skip ahead to the begining of 'The Slippery Slope.'

Klaus and Violet are speeding down a hill in a little caravan, still wearing their 'freak' costume from the previous book, where they dressed up as a two headed person.

Violet says something like "let's get out of this costume" so they can try to figure out a plan to save themselves. the book says nothing about Violets clothes being , in any way, out of the ordinary, and the illustration even shows her in her usual long dress! where the heck did she get that? I guess during the course of a couple books, that piont got over looked. maybe Mr. Snicket realized this to late to say how Violet got her clothes and decided to just write them in rather than have her treck up a mountain in a hospital gown!