Yes, it will be enough. Yeast love to reproduce and pitching a half vial into a quart starter is plenty. Remember than many brewers pitch a full vial into 5 g. of beer without a starter and make fantastic beer. Stepping up the starter should not be necessary.
Also, I like to gently shake the vials, pour a bit off the top into the starter, recap, and only then shake them like a madman to avoid such situations.