I cannot answer your question definitively, but what I can say requires that I be specific about terminology.
As far as I can tell, dim mak is an exclusively Cantonese term. I assume it means or implies: “ to give pinpoint strikes to blood or Qi vessels.” Its presence in English suggests that it was transmitted from practitioners of Southern Styles of Chinese martial arts. Taijiquan is a northern style in origin. Although there are some widely known Taiji lineages that go through Cantonese speakers, I know of none that use Cantonese specific terminology like dim mak.
From what I understand, dim mak is equivalent to the Mandarin term dian xue. Xue refers to acupressure points, rather than to “vessels.” I am not sure what precise techniques these terms refer to, but it has long been known that members of the Yang family have studied techniques that could at least be considered related to these terms.
In Yang Zhenduo’s book, Yang Style Taijiquan, p.7, he describes the type of Taijiquan practiced by Yang Shaohou as originally being like his brother (Yang Chengfu), but gradually changing to a “high ‘frame’ with lively footwork and well-knit small movements, alternating quick and slow actions….The hand movements included catching, pushing and capturing, injuring the attacker’s muscles and harming his bones, attacking the opponent’s vital points and ‘controlling’ his arteries and veins….”
In the Yang Forty Chapters, published in Chinese in another of Yang Zhenduo’s books and in English and Chinese by Yang Jwing-Ming (and also by Douglas Wile?), there are many chapters (28-37) that discuss what some might call dim mak. Neither this term, nor dian xue is actually used, however, but rather four other ones: jie mo (controlling the fasciae), na mai (seizing the vessels), zhua jin (grabbing the tendons/sinews, and bi xue (sealing the cavities).
These chapters are interesting reading, but make a few points that are worth mentioning in the context of a search for information on techniques. They probably explain why there is little mention of these skills in Yang Style materials. First, these are high-level techniques that need a foundation in other skills. Second, secret oral teaching is needed to understand them. Third, they are very difficult to master, even with such teaching. Fourth, they involve life and death and should be taught only orally.
The Chapters also have a list of those types of people who should not be taught and includes a warning about the presence of less than honorable people in martial arts circles. There is really no description of anything that could be considered a “How to learn Dim Mak.”
I hope this helps.