I think you will be fine. According to white labs:
White Labs Pitchable yeast is packaged with 70 to 140 billion yeast cells, which corresponds approximately to a 1-2 liter size starter. Lag times are typically between 12-24 hours for a normal strength brew.
For a 5 gallon brew with ale yeast 150 to 250 billion cells is normal, with up to 400 billion being ok without causing any problems (figures from the same page). Scaling that down to a 1 gallon brew, that's 30-50 billion cells, with 80 billion being a safe upper limit.
After 30 days in the vial, White labs say that yeast viability is down to 75-85% - ca. 50-100 million so we are already close to the safe pitching limit. If the vial was older still then I think there is no reason to worry.
Check the lag time - how long before active fermentation starts after pitching the yeast. If it's more than 5 hours then don't worry about it - the yeast will have needed to bud to increase the population, meaning that your pitching rates were fine to begin with.
If you feel you do need to compensate for over-pitching, you can do this by brewing the same type of wort again and add this to your existing ferment, after racking it to a larger vessel.
Although pitching too much yeast can cause off flavors, these are only ball-park figures - a little more a little less is fine in a homebrew setting. Good temperature control will have a much greater impact on flavor that the small overpitch that you may have here.