Maxinion, I also played a Ventrue. I maxed out my Dominate as quickly as I could.
Mass Suicide was not something that I used too often, but it certainly did come in very handy a few times. I always tried to avoid combat whenever possible, so I would sneak around and hit big groups of enemies with Mass Suicide before they were even aware of my presence. I would usually try to leave one stray baddie so I could refill my blood pool after spending it all on Dominate.
Even after the Warrens you will still face many human enemies, and Mass Suicide can be very useful when you are trying to avoid catching the attention of a hoard of baddies!
And with Suicide, as I'm sure you've noticed, if it doesn't kill the enemy, it at least damages them! I faced lots of bosses where I used suicide and a shotgun to kill them... I'd hit them with suicide as often as I could and fill them with shotgun shells in between. Worked like a charm!
As far as Possession, I LOVE that power! Almost as much as I love Suicide! (That last sentence would sound so wrong taken out of context!

) Possession is great because you can use the enemies against each other, which completely distracts them. You come across a group of baddies, just use Possession on a few of them and watch the fun! As an added bonus, the person you have used Possession on will die after the duration of the discipline expires (if they haven't been killed by someone else first!)
Also, while Suicide and Mass Suicide are not fully effective on a vampire, Possession is! Not 100% of the time, but I'd say there's at least a 50/50 chance of Possession succeeding on a Vampire (but not a boss, of course.) If it doesn't work the first time, hit them again and you'll usually succeed. Then that vampire runs around and attacks the other vampires in the immediate area, and --- here's the best part --- will keel over and die after the Possession runs its course! Possession can be used like Suicide on vampires, but it's so much more useful! At least they can be your tool for a little while before they bite the dust!
If I were you, I would at LEAST drop the 15 experience it takes to get Possession, because that will really come in handy! Mass Suicide is useful as well, but it's up to you whether or not you think it's worth 20 experience. I'm glad that I maxed out Dominate, but if you don't want to spend that last 20 experience in Dominate, drop it into Fortitude! Fortitude can save your butt...