How do i reinstall VPN configuration in my mac?

I presume that you have the VPN settings file from your system admin?

If you received a VPN settings file from your network administrator or VPN service provider, you can just double-click it to set up your connection. If you didn’t receive a file, enter the settings you received manually.

See tis on how to enter the settings manually

1. On your Mac, choose Apple menu  > System Settings, then click Network  in the sidebar. (You may need to scroll down.)
2. Click the Action pop-up menu  on the right, choose Add VPN Configuration, then choose the type of VPN connection you want to set up.
3. Enter a name for the new VPN service in the Display Name field.
4. If you’re setting up an L2TP over IPSec VPN, click the Configuration pop-up menu, then select a configuration.
5. If you don’t need multiple configurations, just use the Default configuration.
6. Enter settings for the new VPN service.
7. Required settings vary by the type of VPN and may include settings such as server address, account name, password and authentication method.
8. If needed, enter additional settings.

• • L2TP over IPSec: You can enter settings for Options, TCP/IP, DNS and Proxies.
  • Cisco IPSec or IKEv2: You can enter settings for DNS and Proxies.

1. Click Create.

See this link for further reference

Set up a VPN connection on Mac – Apple Support (UK)