How do I disable NAT?

You can't actually 'disable' NAT; it's a feature you either use or don't use. The most common reason for asking this question is that users want their public (routable) IP address to be allocated to a client PC (or other device) behind the router rather than that device having a private (NAT) IP address.

Assuming IPv4 is being used, you must use NAT unless you have multiple public IP addresses (a public subnet) or you want your single public IP address to be bridged straight through to another device or router. In those two scenarios, you can avoid using NAT.

Each 'point' on an IP network has to have unique IP address. For example router + client PC would need at least two IP addresses. If your ISP only allocates you a single public IP address then you have to 'create' some more, and we do this usually by using the private subnets (192.168.1.0 etc.). You then have to use NAT to translate one-to-many; to translate from your one public IP address to your many private ones.

If you want to put some other firewall or VPN device behind the Vigor, you can still do this with NAT operating - if you enable the router's DMZ facility to point at the other device then IPSec/ESP and PPTP can both be passed through.

If you don't want to use NAT, you enable IP routing or bridge mode if your router supports them. IP routing and NAT can operate simultaneously.