Would a portable NAT router assist for security when you're travelling? In particular say for the cases:
a) using your own laptop / on an open wifi / using a VPN b) using your own laptop / on an open wifi / no VPN
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If you're using a VPN, the NAT will do nothing more than make things more complicated. Any present established VPN technology is good enough in handshake and encrypting traffic between host and the VPN server that NAT is only an additional bother.
If you're not using a VPN, e.g. using an open WIFI for web access etc., your biggest concern is that anyone can collect and possibly modify your traffic, NAT or no NAT, so it probably doesn't help either.
The only thing NAT will do is make direct connections from another computer to yours a
Edit: Direct connections are possible with many routers/under certain circumstances. There are (parts of) protocols which require a direct connection back to your computer, such as FTP or even IRC. Many routers "help" you by detecting such traffic and doing an automatic port forwarding back to your machine. The tricky part is to convince the router to which ports to open the channel. This is called NAT pinning. Google will tell you more.
NAT is not a firewall nor a security implementation NAT is only for sharing an IP address if you have more than one device having to access public networks, but only one IP.