With bind to character hopefully in place soon (as a item sink) I think it would be a good idea to maybe a lot half the vault reserved for just that character and half to the account. It would make it a lot easier than having to keep track of which item is bound to character within the vault while also giving the character extra space for those items that will be bound to them. This should also give greater importance to houses since your vault inventory is going to be a bit smaller since its broken up like this.
The ratio doesnt really matter but yea just something to think about down the road.