Pitching half a vial to save money is a waste, imo. Here's why - when your beer is done fermenting, you'll have TONS of yeast slurry sitting in the bottom. If you wash it & store it, the little amount left in the vial is trivial. So pitch the whole vial into your starter from the beginning and use less time/steps/DME to build up a large enough cell count.
Proper pitch rates is more about flavor than attenuation; attenuation and fermentation aren't as correlated as one might think. Many an underpitched beer fermented out just fine. Mash temps, grain bills, age of extract, and strain
of yeast affect attenuation more than cell count, IME. When you underpitch the yeast gets stressed and can throw out some unique flavors - sometimes desirable (i.e., hefe), sometimes not, depending on the style.
And you can make awesome beer underpitching - clearly your 1st place showed that. It's just not terribly consistent. And it may have been even better
with a proper pitch.
EDIT: And select intermittent shaking and try to shake it more than 3 times