The biggest reason for NOT repairing a cord this way is because any splice or connection in a wire causes a higher resistance in the wire, thus raising the amperage. If you are using the power saw with a 50’, or 100’ power cord you increase the amperage pull even greater. Amperage causes heat, and heat causes decay. The splice is already weaker than the rest of the cord, and over a surprisingly short period of time the splice will become unreliable at best, extremely unsafe at worst. I also would strongly disagree with using a vac cord as replacement as vacuums are not rated for as high an amperage draw as a power saw. Vacuums use a 16 or even 18 gauge wire i the power cord. A power saw will use 14 gauge at the least.