PokerStars is legal in the US for online casino players aged 21 or older. However, it is only legal in 3 states in the US, these being, Michigan, Pennsylvania, and New Jersey.
With a VPN, you can simply pick a country or a state where online gambling is legal, and you'll be playing at PokerStars, 888Poker, and other online sites in no time. There's more to it a VPN will unblock poker sites when traveling and let you access your account from almost anywhere.