Problem w vamp aura is no matter how many ghouls, how upgraded they are, or what level aura is, ghouls will eventually get 1 shotted. In 1v1 upgrades really slow down tech and dont offer too much benefit. In 4v4 upgrades are more common but there are more enemy heroes that can kill 5 ghouls with one spell. Abombs dont do much dmg and take forever to get.
If you're going vamp you probably playing aggressive early before your ghouls get rocked, so you probably wont have time for many upgrades. If there's time for an upgrade, I would choose +attack. At least aura effects skeletons too.