Your example scenario:
Vamp starts full health (seven boxes):
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0 −1 −1 −2 −2 −5 —
B H I W M C I
Vamp takes 4 bashing.
V20 pg 285
After the soak roll, any bashing damage applied to a vam-pire is halved (round down)
So she only takes 2 Bashing and is now "Hurt". -1 dice penalty.
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Vamp takes 3 lethal. This "pushes down" her bashing. The Vamp is now "Mauled", -2 dice penalty.
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Vamp takes 2 Aggravated. She is now "Incapacitated" and can do nothing. Any further damage sends her into Torpor. Any Agg damage kills her.
V20 pg 283
Further damage suffered by an incapacitated vampire sends her into torpor or, if the damage is aggravated, inflicts Final Death on her.
Let's say the Vamp then took another level of Lethal, sending her into Torpor. We'd still mark the Lethal box above the Bashing, which would "push down" the Bashing damage. The damage in V20 doesn't roll back around, it falls off the track. Here's what she'd look like:
Applying Damage Penalties
V20 pg 282
The number to the left of the lowest marked box indicates your current dice penalty.
So, as soon as you take damage of any kind that marks the "Crippled" box, you are at a -5 dice penalty to any action that would be affected by your injury. I usually read that as any action, since pain usually detracts from concentration as well as mobility. You can give vamps a little leeway with this, since they feel pain less acutely, but that's a Storyteller judgment call.