Got the latest firmware FW101b06 from local Hong Kong distributor, and it fixed the vpn problem.
Before that, I asked taiwan offical technical support too, but they gave me the FW101b05, and it didn't fix the issue.
And I found something interesting, when I open BitTorrent and VPN together, the VPN will be disconnected.
I already disabled the "remote gateway" in my vpn profile, so it isn't the BT traffic cause the disconnection.