If that vapor barrier is kraft paper, that explains why it's falling apart.
Experts say that if you are putting in a second layer of insulation, you should not have a vapor barrier between the two layers. So, at the very least, remove that crumbly vapor barrier and keep the existing insulation in place. I'm not sure how well this will work, though; it might become a laborious process of picking up tiny bits of kraft paper.
You might find that the easiest way to remove the vapor barrier is to just pull out all of the insulation below with it...and then you're back to that first option.
This is not an enviable position, because you're stuck in a kind of limbo. Leave the bad barrier in place, and risk building up moisture. Try to take out the bad barrier, and waste countless hours. If your budget and patience permit, I would strip out everything and start afresh with all-new blown-in insulation (or batt, your choice) in your attic.