Apparently there's a max of 650 vault slots
or 10k days ... or bug?
Anyways, it stopped on 650 and closed vault window, had to re-login to be able to purchase again, but at 31.574.42 / 25 slots expansion, i'll pass
Code
days slots fgs fg/slot/day fg/day
0030 025 0100 0.13333 3.33333
0043 050 0200 0.09302 4.65116
0057 075 0300 0.07018 5.26316
0078 100 0400 0.05128 5.12821
0106 125 0500 0.03774 4.71698
0141 150 0600 0.02837 4.25532
0183 175 0700 0.02186 3.82514
0232 200 0800 0.01724 3.44828
0295 225 0900 0.01356 3.05085
0372 250 1000 0.01075 2.68817
0463 275 1100 0.00864 2.37581
0575 300 1200 0.00696 2.08696
0715 325 1300 0.00559 1.81818
0890 350 1400 0.00449 1.57303
1170 375 1500 0.00342 1.28205
1373 400 1600 0.00291 1.16533
1702 425 1700 0.00235 0.99882
2108 450 1800 0.00190 0.85389
2605 475 1900 0.00154 0.72937
3221 500 2000 0.00124 0.62093
3977 525 2100 0.00101 0.52804
4915 550 2200 0.00081 0.44761
6070 575 2300 0.00066 0.37891
7491 600 2400 0.00053 0.32038
9248 625 2500 0.00043 0.27033
9472 650 2600 0.00042 0.27449