Probably not what you wanted to hear, but for manipulating the vibrate time for a text, you can use a third party app like phonesuite,phonealarm, or a reminder app. I use OS reminder, and it isn't the fanciest app in the world, but it is small, free and does one thing....give me reminders at set intervals of unread texts with a message of my choice and a vibrate at the interval/and length of vibrate time I set.
If interested, it can be found here:
http://www.1800pocketpc.com/2008/07/...fications.html