If you've got another plan for the other half the vial, you'd be fine pitching just half the Brett into your secondary. Like a previous poster mentioned, a lot of people just use what little is left in the dregs of a few beers, so half the vial is probably more than enough. It will take a while to get the brett flavor anyways, so they'll have plenty of time to do any growing they are planning on doing.
That being said, if you don't have any other plans for it I don't think you'd screw anything up by pitching the whole vial. If anything, it might increase the level of brett flavors that develop so you might get to your preferred stopping point a little faster. I'd agree that you don't need much brett to get some funk after a while, but don't see any reason that more would be detrimental.