Does anyone know if booster vaccinations can affect dog's sperm in any negative way - proven dog boostered last week and has a bitch booked to come to him approx end of this week/early next week  - that will be roughly 12/13 days after vaccination. Asked vet the question when the owners of the bitch contacted me to say she had started her season (which was 2 days after the booster), but I got the feeling vet didnt really know.

Hmm, Ive never thought about that. On his vaccination card it should have the make of the vaccine. Try tracking down their number and give them a ring.

Ask them directly and they should be able to give you a more definate answer than your vet.

I cant imagine it affecting the sperm, but it wouldnt hurt to find out.

I've always understood that sperm takes a long time to be affected by the dog's health - for example, a high temperature can cause temporary infertility about 3 months later.

I've always understood that spermatogenesis can be affected by very recent events as, unlike females who's sex cells are all made before birth, sperm are made daily to replenish used or wasted (!) stocks. 
An illness or other condition may temporarily (or permanently) wipe out sperm cell maturation but may only be noticed months later when failure to conceive is investigated.