How do I get the iwatch to vibrate only when txt messages arrive?

Your question is ambiguous, but I think you are asking how to have incoming messages not make an audible alert. In the Watch app on your phone go to Notifications / Messages and toggle the Sound switch to Off.

If your question was to make incoming messages the only alert which has vibration, and everything else doesn’t vibrate, go the same route but for every other application toggle the Haptic setting to Off.