Furnace Defective Vacuum Switch
First make sure that you vents are clear, short and as straight as possible. Both have to be clear, short and straight.  And that have to be the right diameter..Too small--Bad.  And grading. They need a good slope back to the furnace; no sags.
2 As the draft inducer gets older its efficiency drops off.  The fins on the blower wheel have to be clean and sharp.  With just a little bit of tarnish on the wheel it just won't develop the vacuum need to close the vent switch. Also the bearings in the inducer motor dry out with time.  It is subjected to very hot conditions.  High temperatures destroy lubrication. If the inducer assembly is aged or suspect in any, replace it.
Then look at #3:
3 Does jumpering the vac make the furnace work every time.  If it does then you can buy an adjustable vacuum switch.  Dial this in to meet your own needs.
Keep in mind the vac switch is there to keep this furnace safe.  If you defeat its purpose with out meeting its criteria, you could jeopardize the health of the people living in the house.P.