[SteamWolf]

^ Second opinion is always a good idea.
The vapour will vent to atmosphere but it does it via the charcoal canister. The charcoal scrubs out the free hydrocarbons. With the engine running, the vapour is drawn into the inlet manifold without passing through the charcoal portion.

I'd imagine symptoms of a blocked one would be a pressurised fuel tank, possibly leaks at the fuel cap and maybe vapour smells around the car. I couldn't imagine it causing any noticable issues when driving the car, and have never had any problems in my personal experience.