As explained in the doc, with ServerValue.TIMESTAMP you get "a placeholder value for auto-populating the current timestamp as determined by the Firebase servers".

In other words signupTime does not contain the value of the timestamp, since this one is set on the server (the back-end) when you write to the database.

If you want to get this value in your Cloud Function (or in a front-end), you need to query the newly set database node, as follows, for example:

  const signupTime = admin.database.ServerValue.TIMESTAMP

  const database = admin.database();
  const ref = database.ref(`/admin/vendorOrder/${vendor}/${userID}`);

  ref
    .set(signupTime)
    .then(() => {
      return ref.get();
    })
    .then((snap) => {
      console.log(snap.val());
    })
    .catch((error) => {
      console.log(error);
    });