Ken Kilby question is a good one.
First check your square. The knurled nut must be well tightened; verify that the rule can not wiggle. Paul has shown how to check a square.
Otherwise, a “CD jewel box” is a good reference to check a square up to 12 cm.
If it is not square, In the bottom of the groove where the rule slides, there are normally two little blobs of metal. By very very carefully lightly ( I insist very very carefully and lightly) file one of them, it is possible to correct the squareness.
I wouldn’t try to squeeze a plane in a vise; it might snap it.
If there is much material to remove, start with a metal file before using sand paper.
One generally only needs to make one side perpendicular to the sole.