Recently, my ISP is having problem when connecting to a poker site (it's unstable). Would it be legal to use a VPN and connect it to a server in my country when playing poker?
You'll likely have to verify your identity and let them know you're using a VPN. If you setup a private vpn that was hardly ever used publicy, you may not have to do anything.
It very well could be your ISP, try playing at a location near you that has public wifi.
Depends which site you play I think. I know I play at a couple, and one in particular where I ALWAYS make sure my VPN is OFF because they take it very seriously and block you so that you can't access it if it is detected.
Most don't seem to mind, but I definitely don't use it at GGPoker.