Personally I don't have any explanation why the ultimate speed of a true vacuum wavefront would be the speed of light. The best I can do is cite these references (they seem to think so):

https://nerdist.com/vacuum-decay-is-a-very-real-way-all-life-in-the-universe-might-be-destroyed/

https://astronomy.stackexchange.com/questions/8667/the-destruction-of-the-universe-by-a-bubble