I've only seen two vacuum extractions, one in which I was the nurse holding the pump. Here is my take on it....
I was a nursing student with the first one. The mom had an epidural and had been pushing for a whole hour. The nurse and the doc asked several times "Are you getting tired?" Finally the mom (who I thought was doing great despite the epidural) said, "Well, I guess." The doc said, "We can help you out if you want." Then told her they could attach this little vacuum to the babe's head and just pull him out. When the mom asked if it was dangerous to the babe, the doc actually said "Oh, no, not at all." I thought I was going to vomit, I am serious. I had never seen one before, and I couldn't believe how much of the scalp gets sucked into the little cup. I seriously had that rush of saliva, that light headedness that you get right before you pass out or you vomit. The babe, whose strip had looked great until the VE, was born with an apgar of 4.
The second one was in an out of hospital situation. Mom had pushed for a couple of hours (competely on her own accord...when we got there she was grunting in a major way), and for the last hour the babe was right there on the perineum. We kept thinking the babe was coming any minute, but she wasn't. Then her heart rate started to drop; it would drop to 30-40 with contractions, and slowly come up to about 90 between. I am certain it was the babe's heart rate, because I was feeling the mom's pulse with one hand and auscultating with the doppler with the other. The mom's heart rate was higher than the babe's! This is scary as all get out; but we also thought, good lord, this babe is almost out, surely she is about to come!
We were far enough from the hospital and close enough to birth that the doc decided to use the VE. I was shocked that she even had one. She used it during two contractions, babe came out screaming, heart rate immediately shot up to 130s, and she hasn't had any problems since (she is almost one year old now). Her apgars were 9, 9, and 10. Yet all indications were of serious distress. For all we know, she was squeezing her cord with her hand....but by what we had to go on, I had the equipment all set to go to do a complete resuscitation.
I asked the doc about the VE later, and she said she liked it better than forceps for several reasons. With VE, you can't pull harder than the pressure setting; it will pop off the babe's head if you are pulling too hard. Also, if the babe rotates, the VE can be held with one hand, so that your hand can rotate easily with the head. With forceps you can pull as hard as you please, and if the babe rotates, you sometimes lose your grip, or the forceps become misplaced, and can do damage to the babe's face and head. She said it is a rare situation that she would prefer forceps over VE; best of course is not to need them at all.
But like any medical procedure, both have their place. It should be judiciously used, of course, NEVER routinely. I think certain aspects of labor and birth can predispose one to need them....birthing lying down, birthing with an epidural or pain medication, not enough change of position during labor. But I was grateful that day that our doc had a VE up her sleeve, because it was just what that babe and mom needed, and it saved them from becoming a transport. The babe was obviously fine once she finally arrived, but you know that in the hospital she would have been whisked away for observation. I am glad we kept them out of the hospital, but boy, I seriously thought I was going to lose my lunch when that heart rate started to drop and then recover so slowly. We watched the video later, and I have this calm look on my very white face, but my hands are visibly shaking when they are in view. I went home and drank three beers after that, then slept for 14 hours, I was so mentally and physically drained!
Edited for typos