Basically, both things are money in your account.
A VIP Member Credit is the amount of credit added in your account after being charged for your VIP Membership fee any month that you don't shop or 'Skip the Month'. VIP Credits are not breakable. Each VIP Credit must be applied to an entire purchase.
Store Credit is also money in your account, but it can be spent or split in different orders. It will automatically apply at checkout, if available, when placing an order.
Both Store Credit and VIP Member Credits can be combined in a single order.
You can check your VIP Membership and Store Credit balance at the top of the homepage when you're logged in.