Vanishing and Conjuring - how does it work?

[Rach]

No: HPFGUIDX 70715

I have been thinking since reading OOP about the limitations of the 
wizarding world, and how things cannot be fixed with the wave of a 
wand. This led me to wondering how vanishing and conjuring works. 

When Bill vanished the scrolls from the table in Sirius' kitchen 
where did they go? They obviously go somewhere so they can be 
produced again. I'm sure the Order would have needed the scrolls 
back. But what about Snape vanishing Harry's Potion. Is this 
retrievable?  Is there a number of shell-less snails and tail-less 
mice floating around somewhere as a result of McGonnagal's vanishing 
class? 

I have the same confusion about conjuring.  We have lots of examples 
of adult wizards conjuring objects (plates of sandwiches, table 
cloths, squashy purple sleeping bags, chintz armchairs etc) where do 
they come from. It is like summoning - you can only conjure something 
that already exists that you own.  Why can't the Weasleys conjure Ron 
pyjamas that are the right length, or dress robes. 

Can you only conjure something that you have previously vanished? Is 
there some sort of storehouse full of vanished objects from which you 
can conjure stuff perhaps? (Okay I don't really believe either of 
these, but its all I've got). 

Have I missed something in canon that sheds some light on this? I 
think I need a copy of the Standard Book of Spells Grade 5!

Rach