Vibranium isn't indestructible, just very tough and it hasn't been hit with a giant sword wielded by a Titan on the edge before.
In addition, as mentioned over on Science Fiction & Fantasy
Vibranium is the strongest element only on earth!
In Infinity war, we see Corvus Glaive cleave straight through Vision as if he were made of butter.
Considering that, it's not so surprising that Thanos would have the element himself.
Vibranium on Earth is found in Wakanda but the origin of their supply is from a giant meteor crashing there.
It's not unreasonable to assume that Thanos travelling the galaxy would not only have vibranium weapons but potentially even one made of harder metals unknown to us.