pitch at least half, if not the whole vial.
white lab vials aren't enough yeast for optimal pitching of a 5 gallon batch to begin with.
and I believe I've read you need to overpitch by 400% before you need to worry about off flavors.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10