I think most people that would want to "Vent" on the site would need prior knowledge of your site. This isn't something they will go search out when they need it.

I think you should target various Facebook groups and just get the word out. I think a lot of people would be supportive of the site and you could get some viral marketing out of it. Emotional support is in vogue.

There's dozens of niche support websites that would love to know about this. Think forums that offer support for cutters, compulsive eaters, etc.