The size of Vancouver Island is underestimated. It's like a country itself, and it is covered with rainforest, thick rainforest and as the name suggests, it rains a lot! However Vancouver Island is filled with liberal and environmentally friendly people, hippy-like if you will, therefore hitching here is very common. Some parts of the island are quite rural and it can be easy to get stuck after dark in the soaking wet, so make sure to try to get where you're going early on in the day while there's still light.
Nanaimo can be difficult to hitch out of, although there are some decent places to wait, if you can get to them. Nanaimo is quite long, and a long walk may be in order.
If traveling North, get yourself on Island Highway just past Mary Elen Drive. This is just before the Island Highway merges with the 19, which goes all the way North Island. Right after the lights is the slowest, but some traffic will be turning into the strip mall just after the turning lane. Consider waiting just past the end of that lane. The shoulder opens up with plenty of room for cars to stop. They may be a little fast, but you're visible from far away.
If traveling South, get to the interchange of the Trans Canada (#1) and Nanaimo Parkway/Cedar Rd. There's a set of lights , and a large shoulder with a grassy area to wait. When cars get the green, they'll likely be going too fast to stop, but in most cases they'll hit the red. It's not recommended to stand further down, as there's a very fast merge lane off the parkway and a large exit for cars going to the ferry. A sign is recommended here, to avoid confusion about the upcoming merge.
Traveling South: Consider going to Jubilee Parkway and the trans Canada intersection. Although a fair walk, this is the final intersection, so all cars will be going a fair distance. A set of lights slows some cars, and there's plenty of space to wait with good visibility.