Silicone would be ideal. The problem with vinyl is that they fall apart upon removal making them one time use. I've always had no problem with rubber, replacing them every few years, but silicone should last indefinitely. I just searched the web per Kawa's recommendation and found a wide selection of silicone caps of varying thicknesses. The RTV/rubber will also last a very long time. If the carb boots on my XS are doing fine being 30 years old, cracked to high heaven and exposed to the elements a vacuum cap should be a breeze.

 

Step 1: Cut small lengths of the stock vacumme hose. Push a bb (yeah, like from a bb gun), into one end of the short piece of hose. They fit nice and snug and completely block any airflow. Clog port with this little contraption.

Step 2: forgot about it forever