So.... What you do is go to the holdings sieged by the liege of the rebelling vassal and take the holdings yourself. Even with a smallish army, you should be able to assault the holding or the siege will take little time, because he hasn't left many to defend it. You're at war with the liege of the rebelling vassal too. Heck, I go after the recently conquered holdings of the rebelling vassal first. For one thing, it brings down the war score of the liege, limiting the chance of a peace while you are fighting, and for another, those holdings are absurdly easy to capture because they have 30-40 men defending them at most. (I've seen 6, 4, and 1 in defender numbers before.)
Be willing to assault down the holdings of a defending vassal. Best if you can hire mercenaries for the sieging, because they're a unit you'll dismiss as soon as you're done with them, so who cares if 2/3rds of them die in an assault on a heavily fortified castle? But if you can't hire mercenaries, your vassals' levies work just as well as troops whose lives you can throw away without the slightest regret. I've even used my own personal levies. I mean, it's not like their lives mean anything or matter. At all.
Small Edit: I should note that I tend to opportunize like this when the liege in question is eiher:
1) Smaller than me, so he's not a threat
or
2) Occupied with other wars, so his armies aren't a threat