The static pressure of the gas leaving the cylinders is higher than atmospheric. This means that your venturi will need to produce quite a drop to get it much below crank case pressure. It can be done but there are easier ways of providing a crank case breather.
Why not connect the tube from your crank case to the inlet manifold like most car manufacturers do? This also has the advantage of burning the crank case fumes before ejecting them into the atmosphere. Crank case fumes often contain petrol vapours, so introducing them directly into the exhaust might produce a form of (noisy but ineffective) afterburning!!!