The yeast culture in a White Labs vial is in the stationary phase, which means that the cells have low ergosterol and unsaturated fatty acid (UFA) reserves. If the culture has been in storage for a while, then the cells have depleted a large percentage of their glycogen stores, which means that they are a stone's throw away from death. Anyone who has pitched a White Labs vial into a stirred 1L 10% w/v (1.040 S.G.) malt sugar solution can attest that one can expect to at least triple the cell count. If one pitches the starter at high krausen, one will be pitching yeast cells that have sufficient ergosterol and UFA reserves to handle the high osmotic pressure imposed upon their cell walls by a 23.5% w/v (1.099 S.G.) wort.
If the O.P. is willing to perform a stepped protocol, then he can get away with using only one vial of yeast. The first step should be pitched into 500mls of 10% w/v wort (adding 1/16 to 1/8 tsp of yeast nutrient is a good idea). The starter should be placed on a stir plate and incubated for 24 to 36 hours before chilling to precipitate the yeast cells. The supernatant (clear liquid) is then decanted, and the culture is pitched into 1.5 to 2.0 liters of 15% w/v (1.061) wort and placed on a stir plate for 24 to36 hours before chilling to force the yeast cells out of suspension, decanting the supernatant, and pitching the culture into one's batch of beer.