How long can the US Acting President delay becoming the US President?

Question

The US President is limited to 2.5 terms (10 years).
This may be a problem for a Vice-President stepping up, because if he has to take up the post of the US President too soon, even in the best case he will be in office less than 2 terms.

A cunning Vice-President could side-step the issue, being the Acting President for a while and becoming President only later, so he may be in office for all practical purposes for over 2 terms.

Is this even allowed? For how long can the Acting President postpone becoming the US President?

Answer 1

This is the text of the 22nd amendment, the part of the Constitution that deals with this.

Section 1. No person shall be elected to the office of the President more than twice, and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the President more than once. But this article shall not apply to any person holding the office of President when this article was proposed by the Congress, and shall not prevent any person who may be holding the office of President, or acting as President, during the term within which this article becomes operative from holding the office of President or acting as President during the remainder of such term.

Section 2. This article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by the legislatures of three-fourths of the several states within seven years from the date of its submission to the states by the Congress.

See the bolded part. So long as the vice president acted as president for more than 2 years, that person cannot be elected more than one additional time. Whether or not he can become the official president late or not is moot.