President Trump is Mueller’s "primary target"

Is appears now to be when, and no longer if, Trump will have to step down.

Are there any crimes committed by a President, that the Vice-President who succeeded him/her, would not be allowed to pardon him for?

By late 1973, the Watergate scandal escalated, costing Nixon much of his political support. On August 9, 1974, he resigned in the face of almost certain impeachment and removal from office. After his resignation, he was issued a pardon by his successor, Gerald Ford.

https://en.wikipedia.org/wiki/Richard_Nixon

Hopefully If Pence eventually does end up pardoning Trump the GOP will pay a long-term price for their collusion, but I don't hold my breath for that. At least maybe Paul Ryan's presidential aspirations would be trashed in the process.  

Pardon[edit]

Following the release of the smoking gun tape, Nixon's position had become untenable. In his autobiography A Time to Heal, Ford wrote about a meeting he had with Haig before Nixon's resignation. Haig was explaining what he and Nixon's staff thought were Nixon's only options. He could try to ride out the impeachment and fight against conviction in the Senate all the way, or he could resign. His options for resigning were to delay his resignation until further along in the impeachment process, to try and settle for a censure vote in Congress, or to pardon himself and then resign. Haig told Ford that some of Nixon's staff suggested that Nixon could agree to resign in return for an agreement that Ford would pardon him.

Haig emphasized that these weren't his suggestions. He didn't identify the staff members and he made it very clear that he wasn't recommending any one option over another. What he wanted to know was whether or not my overall assessment of the situation agreed with his.[emphasis in original]... Next he asked if I had any suggestions as to courses of actions for the President. I didn't think it would be proper for me to make any recommendations at all, and I told him so.

— Gerald Ford, A Time to Heal[4]

Ford later decided to pardon Nixon for other reasons, primarily the friendship he and Nixon shared.[5]

President Ford announces his decision to pardon Nixon, September 8, 1974, in the Oval Office

Following his resignation on August 9, 1974, the Nixons flew to their home La Casa Pacifica in San Clemente, California.[6] According to his biographer, Jonathan Aitken, after his resignation, "Nixon was a soul in torment".[7]Congress had funded Nixon's transition costs, including some salary expenses, though reducing the appropriation from $850,000 to $200,000. With some of his staff still with him, Nixon was at his desk by 7 a.m.—with little to do.[7]His former press secretary, Ron Ziegler, sat with him alone for hours each day.[8]

Nixon's resignation had not put an end to the desire among many to see him punished. The Ford White House considered a pardon of Nixon, though it would be unpopular in the country. Nixon, contacted by Ford emissaries, was initially reluctant to accept the pardon, but then agreed to do so. Ford, however, insisted on a statement of contrition; Nixon felt he had not committed any crimes and should not have to issue such a document. Ford eventually agreed, and on September 8, 1974, he granted Nixon a "full, free, and absolute pardon", which ended any possibility of an indictment. Nixon then released a statement:

I was wrong in not acting more decisively and more forthrightly in dealing with Watergate, particularly when it reached the stage of judicial proceedings and grew from a political scandal into a national tragedy. No words can describe the depth of my regret and pain at the anguish my mistakes over Watergate have caused the nation and the presidency, a nation I so deeply love, and an institution I so greatly respect.[9][10]

 

https://en.wikipedia.org/wiki/Pardon_of_Richard_Nixon#Pardon