Perhaps you should READ a contract and the Acceptable Use policies before you sign up for service with the intent to DEFRAUD the company.
That's what you are doing. If you don't pay for the tethering and you try to get around it, you are violating your contract, and abusing the system.
And, it's people that do things like this that make it MORE EXPENSIVE and harder for people like the rest of us that follow the rules.
Using a VPN or any other method won't help you. AT&T uses the HOP counts in the TCPIP headers to determine where the traffic is coming from. Every device along the connection adds another hop to the headers. AT&T knows that your device (phone) should be the next hop from the WAP. If it see's the data going 2 hops past the WAP (Your device, THEN the tethered device), they know it's not stopping at your phone.
As far as anyone here offering you HELP, I can tell you that you won't get it. It is a violation of the rules you agreed to when you joined here. We can't tell you how to violate your contract. That would make this site just as Liable for the violation as you are.
Sorry - but it's like this. If you want to tether, you pay for a tethering plan.
I can tell you what I did. I picked up a Mobile WiFi hotspot that's Pre-Paid. (You can get them at Wal-Mart for about $100 for Virgin, AT&T or Verizion.) When I'm going to be on the road, I pay for the service I use and my Laptop and tablet can both surf on it. If I'm not going to be on the road and need it, I don't pay anything.