From Vita Dev Wiki
You can also become friends with someone using: a PS Vita TV system, PS3™ system, or PS4™ system.
The friend request will be sent to all devices the friend's account is used on. A maximum of 2,000 friends can be added.
In some applications and games, a maximum of 100 friends are displayed. If you have more than 100 friends, you must sign out and then sign in again to update the friends who are displayed in games. When doing this, also sign out of all other devices that you are signed in to PSN on.
View information about yourself, and other players such as your friends and players with whom you have played online games.