The original vacuum switch lasted 11 years on one of my own boats...failed in 3 years on the next boat.

It's not a preventive maintenance item...there's nothing to be gained by replacing it before it fails, nor is there any need to keep one on hand unless you're planning to leave the country for an extended period...'cuz the toilet will work just fine if the vacuum switch fails, you just have to turn it on and off at the breaker (one of the good reasons to have a night switch in the head) till you replace it. And it's an easy repair that only takes about 15 minutes.