Minor Traitor chaplains can force monkeys to sign contracts

[Fenriswolf1991]

There is even another way for vampires to use monkeys, without having to humanify them first. Just stick some IV drips on a monkey and wait, then take it back when it´s full. This "canned" blood nets less blood points than fresh blood, but it´s an alternative and you most likely can do this relatively safe and unseen. 

You just have to be careful if the monkey has some chemicals in his bloodstream, because you will get them too with the IV drip.