Little Froggie: “LuL: I don’t think it’s a Mitzvah to vent.. it’s rather a Mitzvah to lend an ear and lend a shoulder to someone venting… (if I’m not mistaking)”
I’m pretty sure it works both ways. I don’t have a source off-hand, but I asked Rav Pliskin (author of “Guard Your Tongue”) about it, and he told me that it is muttar to speak Loshon Hora in order to vent (obviously you have to be careful to fulfill the conditions of “toeles” and choose the person you speak to wisely).
In this case, there is no loshon hora issue, so it’s certainly not assur.
In terms of being a Mitzvah, I don’t have a source on me (although I can ry to come up with one when I have time) but I am pretty sure it is a Mitzvah. It is part of taking care of your emotional well-being, which is the reason one is allowed to listen to someone vent.