Your example scenario:
Vamp starts full health (seven boxes):
[ ][ ][ ][ ][ ][ ][ ]
0 -1 -1 -2 -2 -5 --
B H I W M C I
Vamp takes 4 bashing. She is now "Wounded". -2 dice penalty.
[/][/][/][/][ ][ ][ ]
Vamp takes 3 lethal. This "pushes down" her bashing. The bashing damage in V20 doesn't roll back around, it falls off the track. This is enough to "Incapacitate" the Vampire.
[X][X][X][/][/][/][/]
Vamp takes 2 Aggravated. Well, an Incapacitated Vamp is immediately killed by any Agg damage
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.
But, for argument's sake, let's say that your example Vamp was sitting at
[X][X][X][ ][ ][ ][ ]
and took two points of agg:
[ * ][ * ][X][X][X][ ][ ]
They'd be "Mauled". -2 dice penalty.
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.