What is the difference between vm.$set and Vue.set?

Here is the release note that went with the introduction of Vue.set:

Vue no longer extends Object.prototype with $set and $delete methods. This has been causing issues with libraries that rely on these properties in certain condition checks (e.g. minimongo in Meteor). Instead of object.$set(key, value) and object.$delete(key), use the new global methods Vue.set(object, key, value) and Vue.delete(object, key).

So, .$set used to be available on all objects - it is now only available on a View Model itself. Vue.set is therefore used in those cases now when you have a reference to a reactive object but do not have a reference to the View Model it belongs to.

In simpler terms, Both are same, $set available inside component(instance) like this.$set(), where as Vue.set is static method available globally.

this.$set(someobject, 'key', 'value')

Vue.set(someobject, 'key', 'value')