A D3DVECTOR is a Direct3D structure which is used to define a vector.
The structure definition contained in the D3DImmediate section of the DirectX 6.0 SDK is:

typedef struct _D3DVECTOR {
union {
D3DVALUE x;
D3DVALUE dvX;
};
union {
D3DVALUE y;
D3DVALUE dvY;
};
union {
D3DVALUE z;
D3DVALUE dvZ;
};
} D3DVECTOR, *LPD3DVECTOR;