i hhave bought a vault if i add 50 slota to it does that raise next months monthly fee or do i hace to purchase 50 slots again next month or do i just pay 50 fg for all 75 slots

Only if u have 25+ items

Keep it at 25 if your fg starved

YOu will have to pay what ever is required to hold the number of items in your vault
if you have 25 items in your vault or less you will only have to pay for 25 spaces, but if you have 26 items you have to pay for 50 spaces

At your level of play you have no need for more than 25 slots. Just utilize mules.