Does carbon dioxide cause vasodilatation or vasoconstriction?

It causes vasodilatation. Remember that the blood flow to tissues is largely controlled by local factors. (This contrasts with blood pressure, which is centrally controlled.) As tissues consume energy, their waste byproducts tend to cause vasodilatation. This is rather clever, since it means that those tissues working the hardest will receive the most vasodilatation, and hence the most new 'energy' substrates (like glucose and oxygen).

Carbon dioxide is a well-recognised product of aerobic metabolism (for instance, glucose is eventually aerobically metabolised to yield energy, carbon dioxide and water). Thus, carbon dioxide causes vasodilatation, in accordance with the rationale above. This is one reason why you can get a headache if you don't get enough fresh air in (e.g. by rebreathing the same air in a plastic bag) - the carbon dioxide dilates the cranial vessels.

One exception to this rule comes in the lungs, where the response to carbon dioxide is the opposite - it causes vasoconstriction. This is to try to preserve the lung's ventilation-perfusion (VQ) ratios. This is another whole topic in itself, and will have to be left for another day. The wikipedia article is here.