I owned both and use Vana for SP intensive leech, while RWH for HP intensive leech.
Vana chance never disclosed in DB but the sure thing it got higher % chance to absorb SP than RWH does based on my personal experience and lots of other people claims based on their experience too.
as for HP the damage modifier % absorbed as HP is higher in RWH 16% damage absorbed, while vana only 8% damage absorbed also it's easier and cheaper to boost hp leech percentage than sp leech percentage
This topic has been argued so many times