Vaccines are not effective against trypanosomiasis because

The organism can change its protein coat and thereby evade the immune response.
Hope it helps :)

Vaccines aren't effective against tryoanosomiasis because it can actually hurt your amune system if the parasite changes it's coat! That can do actually more damage than not - so definitely not the most effective treatment. Just to give you a tad more background on the topic, tryoanosomiasis is actually referred as the "sleeping sickness" for the more obvious reasons such as the inability to get on a decent sleeping pattern because of all of the symptoms working their "magic." This is a parasitic disease that can also affect animals. It may not be the most effective but - it's best to seek out treatment because IT CAN be fatal if not treated properly!
I hope this helped, and you pass! (: Good luck, rockstar! (: