Gold Vault Strategies

I recommend making and upgrading one large gold vault rather than 3 small ones.  It's easier to protect one vault compared to 3.  Mine is nestled amongst 3 arrow towers.

Once it is big enough, you can develop the other vaults and use them as part of your wall to protect the one that matters.

In terms of upgrading vaults and mills and mines, try to balance between the mana and the gold because each depends on the other for upgrades.  Failing to upgrade on either side can leave you lopsided.  Keep your balance!