Is the idea that a Pope may be anti-Christ, the same as basically saying the chair of St. Peter is vacant?
Chair of St. Peter refers to an actual chair, but also the spiritual leadership. In 2012, Pope Benedict XVI described the chair as "a symbol of the special mission of Peter and his Successors to tend Christ’s flock, keeping it united in faith and in charity." -source-
A vacant chair is referred to sedevacantism.
Anti-Christ is defined as one who opposes Christ and substitutes itself.