depending on what vents your talking about, the round or straight ones:
Round be very careful you touch some and they break up like dust (found this out the hard way) now you dont have to.
Straight one I have never had a problem with as these are new for now...
If you break round vents replace them or highlght this before you start so you don't have to, I have insurance to cover us to do this job, I charge very little for this highly skilled job

, you get the vents from wickes or gibbs and dandy about £1 each..... can should be able to see an add on here guys

stanley knife or simalar is required as 9/10 the vents are not same hole size by 1mm or 2 :

Washing them in winter is easy summer is a cow, I will say the way I clean them in winter
Spray tfr on all round, go back to start and using wfp brush cleaning them finish one side at a time. Rinse next side same , clean windows, then wrap a micro fiber on brush with a rubber band and dry off the hangging drips. jobs a good'un

I have never had a complaint from doing it this way