Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| State | Type | Name | Title | Royal house | From |
| --- | --- | --- | --- | --- | --- |
| Cai | Sovereign | Yi | Marquis | Ji | 837 BC |
| Cao | Sovereign | Dai | Count | — | 826 BC |
| Chen | Sovereign | Li | Duke | — | 831 BC |
| Chu | Sovereign | Xiong Xun | Viscount | Mi | 821 BC |
| Jin | Sovereign | Xian | Marquis | Ji | 822 BC |
| Jin | Sovereign | Mu | Marquis | Ji | 811 BC |
| Lu | Sovereign | Wu | Duke | Ji | 825 BC |
| Lu | Sovereign | Yi | Duke | Ji | 815 BC |
| Qi | Sovereign | Li | Duke | Jiang | 824 BC |
| Qi | Sovereign | Wen | Duke | Jiang | 815 BC |
| Qin | Sovereign | Zhuang | Duke | Ying | 822 BC |
| Song | Sovereign | Hui | Duke | — | 830 BC |
| Wey | Sovereign | Li | Marquis | — | 855 BC |
| Wey | Sovereign | Wu | Duke | — | 812 BC |
| Yan | Sovereign | Li | Marquis | — | 826 BC |
Question:
When was Xiong Xun a vassal?
Answer:
821 BC