If the vapor's volume were to be incorrectly recorded as 125ml, how will this error affect the calculated molar mass of the unknown?

Since you didn't give the actual volume (or any of the experimental values) I can only tell you how to do it. Do the calculation using the real (determined) volume of the flask. Then, re-do the calculation with v = 125ml. Take the two values and calculate % error; m = measured vol; g = guessed vol.

[mW (m) - mW (g) ]/mW (m) x 100%

(they want % error so, if it is negative, just get rid of the sign)