Yama is a known God of the lower realm in all forms of Buddhism, I am not knowledgeable about particularly Vajrayana, but I guess they do accept him, there is no reason why not, when he mentioned in all of the sutras
Yama is the God that humans meets when they die, because he rules Karma
I am not sure whats your issue with that
Hope this help
When you say "Vajrasatva purification practice" do you mean an appropriate visualization plus the hundred-syllable mantra followed by a dissolution?
If so then the answer is: no, this is not training in the Ultimate Bodhicitta, this is a preliminary practice.
If by "Vajrasatva purification practice" you don't mean the above mentioned ...