Vacuum pipes

Hi all... after a shortish run today i developed a misfire... i thought perhaps the vacuum pipe i had problems with before (from distributor to F.i. intake has come loose again.... but no.. at tick-over it was virtually at a stall...  i popped the bonnett...and after a little bit of poking around the vacuum pipes it cleared itself and sat idling at an even tick-over... i had while it was running rough pushed and squeezed some of the pipes (vacuum) near the exhaust manifold and the block (sensor?)  i got to thinking perhaps it might be wise to replace all these pipes in one go.... so to my question...

do i just buy a long vacuum pipe from a motor factor(and cut to length)...or are they a specific bore? should i buy MB ones?  i suspect they are going soft and leaking a little...unless my misfire was dirt in the injector...it did sound like it wasn't firing on all six... all plug leads etc checked.. none loose..

i had just ran her at 65mph... (3/4 full fuel tank)

no apparent splits or leaks in the pipes... could the sensors be faulty or the diaphragm that sits on the manifold?

advice please..