In the Vault building you can deposit gold to get it back later with interest.
There are various schemes that each have their own term and interest rate. You can deposit between 1000 and 1500 gold and after the term expires you can collect the deposit plus specified interest.
The scheme with the best yield is also the only of the four schemes that is only available once you have purchased a pack. This scheme has a term of 1 Day and an Interest Rate of 7%. This means if you deposit 1500 Gold you can get 105 gold per day.
The other Schemes are 7 days for 7% (105 gold per 7 days , or 15 gold per day, if you invest 1500), 15 days for 20% (300 gold for 15 days, or 20 gold per day, if invest 1500), and 30 days for 50% (750 gold for 30 days, or 25 gold per day, if invest 1500)
If you purchase a pack the best Scheme is clearly the one day deposit. Its yield is over 4 times better than the best of the rest. Still, 105 gold per day is not huge, but if you are in it for the long run it is for sure worth considering to purchase a small pack early on, so that you can keep benefiting from the steady stream of gold for a long time.
If you decide not to purchase and are still in the game for the long run, the best investment is to go for the 30 day deposit. It yields the most gold per day.
This is no get quick rich scheme, but it does offer a nice amount of gold every day, especially if you decide to unlock the 1 Day deposit by purchasing a pack.