From Vita Dev Wiki
Messages can be sent and received only between players on other PS Vita systems, PS TV systems, PS3™ systems, PS4™ systems, and players using smartphones or other devices with PlayStation®App installed.
You can send a message to a group of 1 to 99 users. Ask the message recipients in advance for their online IDs. You can enter up to 512 characters in a message.
A maximum of 200 messages are displayed. After you reach this 200 message limit, your oldest message will be automatically deleted each time you send or receive a new message.