Mine

static void VertexBufferDescTest(void)
{
    HRESULT rc;
    D3DVERTEXBUFFERDESC desc;
    union mem_t
    {
        D3DVERTEXBUFFERDESC desc2;
        unsigned char buffer[512];
    } mem;
    memset(&desc, 0, sizeof(desc));
    desc.dwSize = sizeof(desc);
    desc.dwCaps = 0;
    desc.dwFVF = D3DFVF_XYZ;
    desc.dwNumVertices = 1;
    rc = IDirect3D7_CreateVertexBuffer(lpD3D, &desc, &lpVBufSrc, 0);
    ok(rc==D3D_OK || rc==E_OUTOFMEMORY, "CreateVertexBuffer returned: %x\n", rc);
    if (!lpVBufSrc)
    {
        trace("IDirect3D7::CreateVertexBuffer() failed with an error %x\n", rc);
        goto out;
    }
    memset(mem.buffer, 0x12, 512);
    mem.desc2.dwSize = sizeof(D3DVERTEXBUFFERDESC)*2;
    rc = IDirect3DVertexBuffer7_GetVertexBufferDesc(lpVBufSrc, &mem.desc2);
    if(rc != D3D_OK)
        skip("GetVertexBuffer Failed!\n");
    ok( mem.desc2.dwSize == sizeof(D3DVERTEXBUFFERDESC)*2, "Size returned from GetVertexBufferDesc does not match the value put in\n" );

    ok( mem.buffer[sizeof(D3DVERTEXBUFFERDESC)+1] == 0x12, "GetVertexBufferDesc cleared outside of the struct!\n");

out:
    IDirect3DVertexBuffer7_Release(lpVBufSrc);
}