1. Can evil monks take vows? I think it's intended for good but it doesn't explicitly say so.
2. If following the vow would be way easier than normal for the monk in question (such as an asexual monk taking Vow of Chastity, or a monk who of a race that doesn't eat anything taking a Vow of Fasting), do they still get all the benefits? Or is this cheating?
1. Can evil monks take vows? I think it's intended for good but it doesn't explicitly say so.
Vows don't have anything to do with the good/evil axis. They represent the lawfulness that's mandatory for Monks.
2. If following the vow would be way easier than normal for the monk in question (such as an asexual monk taking Vow of Chastity, or a monk who of a race that doesn't eat anything taking a Vow of Fasting), do they still get all the benefits? Or is this cheating?
Yes. Because neither of these actually lessens the mechanical downsides of the vows. Vow of Celibacy goes way beyond sexuality, it prevents touch spells and sleeping in proximity to others. Similarly, the main thign about Vow of Fasting is that it prevents using potions et. al., the rest ist just roleplay stuff.