Cisco core switch WS-C4507R

I have a WS-C4507R Core switch and 4 routers connected to it and Many WS-C2960-48TC-L also connected to it.

One of my routers (172.30.7.254) have vpn over ip link to other city, and there other 3 routers are connected to 3 ISP’s

My users divided into three groups each group browse INTERNET from One ISP.

When any user need any resource from the other city network he should change his gateway manually to 172.30.7.254 and to browse INTERNET he should change it back to ISP gateway.

I need to program the core switch so that it check the request when the user need INTERNET it direct him to the required ISP gateway and when user need Local resource from the other city network it direct him to the 172.30.7.254 gateway?