Vastly means by an enormous amount or to an extreme degree. When two things are vastly different, they're separated by a huge gap. A bicycle and a spaceship are vastly different machines. Your knowledge of fractions now is vastly greater than when you first started learning them.

The word emphasizes the scale of difference or change. A student who studies hard might improve their spelling grade, but if they go from barely passing to winning the school spelling bee, they've improved vastly. The Pacific Ocean is vastly larger than even the biggest lake. The number of stars in the universe is vastly greater than the number of grains of sand on all Earth's beaches.

You'll often see vastly paired with words like “superior,” “different,” “improved,” or “underestimated.” It signals that we're talking about differences so large they change everything. When scientists discover a galaxy vastly farther away than any previously known, they're describing a distance almost too large to imagine.