take out your cell phone while heading to your car if it's nearby, call 911, and snap a pic of them and the car and send it to the police. Then follow with your car, keeping the police updated on where they are located.
If you don't have a car, you could do everything the same (just take the pic while walking away from them a bit), then either A: Try and take them out/slow them down (especially if there are things to throw around, just get them chasing you and not holding the woman....possibly giving her a chance to escape), or B, just send the info and report the direction the van is traveling in for as long as you can see it (possibly trying to flag down any other car and get them to follow it).
Just charging the guys is an option if you don't have people your responsible for, especially since only one is outside the car and just has a knife, almost anyone could probably distract him enough for the girl to get away, and with decent martial arts training you might even be able to take him out.
After which (or even while doing so) try and make sure there is something between you and the van, in case someone inside has a gun.
Then wait with the chick to the police arrive, and deal with any idiots that come out of the van after her (even with a gun if your behind a car, they can't really shoot you till they get within h2h range.
Still risky, gotta weight the risk (to other people your responsible for more then yourself) and do what is best for everyone in that situation. Also can look for others around to alert to the situation, one person alone is one thing, but if 5-6 bystanders get involved they may just cut and run (lets just hope they don't cut the girl and run though).
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