I don't think the vault holds ANY of your silver until higher level...maybe 22 or 23.
If your OTHER resources went below 200k, then yes, that'd be a bug.
in the screeshot, we can see that you have a castle lvl 7... but we don't see the lvl from your vault...
requirement for castle 6 is wall 5 / vault 5
requirement for castle 7 is wall 6 / infirmery 6
So, it is possible to have a castle lvl 7 with a vault lvl 5... vault lvl 5 have a capacity of 150k only... a lvl 6 have a capacity of 175k, who seem to match the rss who was taken...