it may be the vest! i find that if sophias vest is done up and folds in by her legs, it wicks the wee out of the nappy resulting in leaks. If i can help it i try to leave her vest unpoppered, or get bigger vests.
The legs also may be to tight, has anyone said that already? I have just been reading on another forum, about pockets leaking and a couple of people have said when they loosened the legs they stopped leaking. Doesn't make much sense to me, but if it works it works right!