Mike Pence seems suddenly to have realized that Trump can’t fire him
Although the President can name a new Veep to fill a vacancy (with majority approval of both houses of Congress), the President can’t remove the VP from office. To do so requires the same impeachment/conviction process as removing the President himself.
In fact, it’s actually easier to remove a President than a VP. Although either can be impeached, the President can also be removed through the 25th amendment, while there is no similar process covering the Veep.