How do you figure your basis? Your per share basis is the same per share "fair market value" used by your employer to calculate the compensation created by the vesting. Assuming that "27555" is the compensation created by the vesting as reported on your W-2 simply divided that number by the number of shares that vested. (NOT the number of shares you received after shares were withheld or sold for taxes.) Multiply that number by the number of shares sold and you've got your basis.
A same day sale typically results in a small loss due to selling commissions and fees.