Vitest has been designed with a Jest compatible API, in order to make the migration from Jest as simple as possible. Despite those efforts, you may still run into the following differences:
Globals as a Default
Jest has their globals API enabled by default. Vitest does not. You can either enable globals via the globals configuration setting or update your code to use imports from the vitest module instead.
If you decide to keep globals disabled, be aware that common libraries like testing-library will not run auto DOM cleanup.
Auto-Mocking Behaviour
Unlike Jest, mocked modules in <root>/__mocks__ are not loaded unless vi.mock() is called. If you need them to be mocked in every test, like in Jest, you can mock them inside setupFiles.
Mocking globals
In Jest, you could mock global variables by putting them on window object. Vitest treats globalThis and window as different objects, so you would need to either put it on both window and globalThis, or use vi.stubGlobal helper.
Jasmine API
Jest exports various jasmine globals (such as jasmine.any()). Any such instances will need to be migrated to their Vitest counterparts.
Envs
Just like Jest, Vitest sets NODE_ENV to test, if it wasn't set before. Vitest also has a counterpart for JEST_WORKER_ID called VITEST_WORKER_ID, so if you rely on it, don't forget to rename it.