In light of the fact that the Victor's inputs are highly discretized by the Victor itself (see
http://www.vexforum.com/showthread.php?t=20350), a small lookup table (mapping PWM duty cycle<->speed) will ultimately provide the best possible accuracy, and will be computationally inexpensive to run. If you look under the hood of the WPIlib PWM code, all continuous-valued speed commands are ultimately converted to a PWM duty cycle value between 1 and 255 (0 is a reserved value for holding the line low). The effective duty cycle for the Victor is then .665ms + X*6.525us, where X is the PWM value. Since there are only 255 unique values, at most you need 255 elements in your lookup table (and in actuality, the beginning and end of this list will be past the Victor's 1.0 to 2.0ms allowable input range spec anyhow).
Define a float/double array with 255 values. The index is the PWM value; the value is the actual speed for that value. To populate this array you could, for example, run a "sweep" of the Victor by running a motor at each possible value for a couple of seconds and recording the total distance traveled during each segment (you would want to filter this data to preserve monotonicity). It would take several minutes to run, but you only need to do it once (when I get a chance, I will take an old bot and do exactly this, and post the results).
Driving the Victor would be accomplished by searching through the 255 value array for your desired speed and selecting the index of the closest value in the array. Using a nearest-value binary search (since the array ought to be sorted) would require only 9 comparisons*, worst case, to select the optimal PWM value from the array.
Using this method, you are guaranteed to have the least possible linearization error, though the polynomial method is close enough that I doubt you'd notice the difference.
*At most 8 comparisons are necessary to search a 255 element array (since worst case complexity is O(log2(n)), but you would want an extra comparison to check if the adjacent value in the array is actually closer, since the easiest way to write a nearest-value binary search would be to find the closest value that is always either > or < the input.