Review section one of your private pilot ground school book, or this Wikipedia page (https://en.wikipedia.org/wiki/Pitot-static_system).  Your airspeed indicator has nothing to do with your vacuum system.  (Now you're starting to realize why understanding that stuff might be important!)

Your airspeed indicator relies on the pitot and static ports.  If the airspeed indicator works the next time you fly on a dry day, then the rain was probably the culprit.

What was the outside air temperature when you were flying?  (If it was cold enough, you could also have been picking up ice.)  Do you have Pitot Heat in your plane?  Might have helped...

Frequent cause for this is insects stopping up the pitot tube.  Your A&P can disconnect the hose to your IAS and blow it out with compressed air and likely fix your problem.