I dont think there is any way to erase only wireless part.
You can take a back up VPN config and once router is back to default just paste back the VPN part !
This is how you can reset it.
Q. What is the use of the reset button on Cisco 870 and 850 Series routers?
A. The reset button is used to restore the router to the default factory settings if pressed within 5 seconds of router power up. In line with this implementation, the following scenarios are possible:
? The router will not react to the reset button if pressed after the 5 seconds of power up.
? When the reset button is pressed within 5 seconds of boot up and there is no valid xxx.cfg file in the flash memory, the router boots up with the factory defaults.
? When the reset button is pressed within 5 seconds of boot up and there is a valid xxx.cfg file in the flash, the router boots up with the xxx.cfg file and avoids the startup-config file in NVRAM.