READ THIS BEFORE USING BORAX...from about chemistry . com
Borax is natural, but that does not mean it is automatically safer for you or for 'the environment' than man-made chemicals. Although plants need boron, too much of it will kill them, so borax can be used as an herbicide. Borax may also be used to kill roaches, ants, and fleas. In fact, it is also toxic to people. Signs of chronic toxic exposure include red and peeling skin, seizures, and kidney failure. The estimated lethal dose (ingested) for adults is 15-20 grams; less than 5 grams can kill a child or pet. For this reason, borax should not be used around food. More commonly, borax is associated with skin, eye, or respiratory irritation. It is also important to point out that exposure to borax may impair fertility or cause damage to an unborn child.
You will have to throw out the vacum bag AND clean the filter each time you vacum.
Unfortunately I don't know if the borax & salt will truly work to eradicate them. I tried borax, not salt, and when we had a flea problemand I did have to resort to using flea bomb after a couple of months.
GOOD LUCK.