Vlads is a supportive item in nature. IMO, works better on Veno than Ursa, since it'll just give Ursa an item slot for a better choice.
At higher levels, a Void Stone provides more than enough mana regen. You don't even get the lifesteal from vlads as venomancer.
Venomancer has no orbs, thus giving him lifesteal. Even if he did have an orb, Vlads is not, making it so he would have lifesteal.