To change the maximum allowed messages stored in the Inbox by your Vbulletin members you’ll need to go to.

Open your admincp

And go to:

Usergroups > Usergroup Manager > Registered Users > Private Message Permissions > Maximum Stored Messages

You might want to change it for each user group. So click on each user group and change it accordingly. You will need tp scroll down in the Registered Users and find Private Message Permissions and change the Maximum Stored Messages: field.

By default in Vbulletin 4 the maximum allow private messages in 50.