So I finally got around to looking at vector's in the AVM2 yesterday (for asmock) after someone logged a bug and found that the Flash 10 features are not actually documented in the AVM2 Overview document. Fortunately, after some digging around, I uncovered enough to implement support. Here's what I found:

References to a typed generic type (Vector. as opposed to Vector.) are referenced by a new multiname kind (0x1D), which I call GenericName. GenericName has a format like so:

[Kind] [TypeDefinition] [ParamCount] [Param1] [Param2] [ParamN]

[TypeDefinition] is a U30 into the multiname table
[ParamCount] is a U8 (U30?) of how many parameters there are
[ParamX] is a U30 into the multiname table.

Obviously generics are not generally supported yet, so ParamCount will always be 1 (for Vector.).

The other interesting thing is how instances of the class are created. A new opcode was added in Flash 10 (0x53), which I will call MakeGenericType. MakeGenericType is declared with the following stack:

TypeDefinition, ParameterType1, ParameterTypeN -> GenericType

It also has one parameter, a U8 (U30?) specifying how many parameters are on the stack. You will generally see MakeGenericType being used like this:

GetLex [TypeDefinitionMultiname]
GetLex [ParameterTypeMultiname]
MakeGeneric [ParamCount]
Coerce [GenericNameMultiname]
Construct [ConstructorParamCount]

So if you had the following...

GetLex __AS3__.vec::Vector
GetLex int
MakeGeneric 1
Coerce __AS3__.vec::Vector.
Construct 0

You would now have an instance of Vector.