Simple answer is they WILL fail. Change them now and save yourself a lot of trouble.

When the vac hub seals fail they draw a lot of water and dirt into the wheelbearing area, or of the hoses fail the water and dirt can seep into the hubs. Another possibility is the suction could draw water/dirt up into the intake manifold and into the cylinders (although that has not been reported).

Basically the vac hubs are <img src="/forums/images/graemlins/evil.gif" alt="" />, you are going to get left <img src="/forums/images/graemlins/confused.gif" alt="" /> as to why you are all of a sudden going nowhere. Change them now <img src="/forums/images/graemlins/kewl.gif" alt="" /> and say <img src="/forums/images/graemlins/nana.gif" alt="" /> to any possibility of <img src="/forums/images/graemlins/shiner.gif" alt="" />. You can then quite happily show people your <img src="/forums/images/graemlins/butwiggle.gif" alt="" /> as you go past them.