Yes, that is only 55% attenuation. 6 days is a little too early to judge 100%, but chances are that if you didn't build a starter, you underpitched and it could have a problem reaching the level you hoped.
Pitching another vial won't do a heck of a lot as it is a very hostile environment to throw fresh yeast (high alcohol, low PH, no oxygen). I'd wait until after day 10 and see where it is at. If it is still having issues, make a starter and pitch the whole thing into your beer while it is at high krausen. This could help with the attenuation.