Why Herakles ?
Now we know why artists 4000 km East of Greece knew about its gods and heroes. But using Herakles as a template for Vajrapani was not the most obvious choice considering the Greek pantheon of gods and demi-gods. There are more important gods and above all gods that hold thunderbolts in Greek myth, Herakles had no thunderbolt.
Herakles is among the scope of Greek deities that were assimilated by the Kushans. Kushan rulers, who probably had no direct links to Greece (Greece was now under Roman control), still identified with this Demi-god as we can see on some of their coins. The Herakles myth was still strong in Kushan times and many architects, acrobats, athletes and craftsmen of Greek decent still worked in the Kushan empire.
Alexander the Great made the link between his kingship and his own decent from Herakles. This idea that Herakles equals kingship was continued by subsequent kings be it Greek or Kushan. Perhaps Vajrapani’s frequent appearance as Herakles in art made it easier for rulers to support this demi-god it would after all include a symbolic representation of their own kingship. “The powerful patron and defender of Buddhism that is always on the Buddha´s side.”,
Merchants were also important donors to the temples and monasteries and might also have felt an analogy with Herakles the travelling demigod who had to overcome difficult tasks during his lifetime.
Besides the Kingship argument there is one other important analogy in the myth of Herakles and that of the Vajrapani of the Pali texts, Herakles has the title of; “Alexikakos,´averter of evil´, because he purged the world of so many monsters and evildoers.” (ref. The Penguin Book of Classical Myths by Jenny March ) Vajrapani also purges evildoers, not by killing but by converting them to Buddhism.
These two arguments might explain why Herakles was a popular candidate for the role of Vajrapani, but why did Vajrapani appear always next to the Buddha? In the Buddhist text Vajrapani only appears a view times. For some reason Gandhara artists included Vajrapani in most scenes right next to the Buddha as if he were a fixed companion of the Buddha.
Historically Vajrapani in his Herakles appearance of a strong muscled man is described as a bodyguard or personal protector always at the side of the Buddha. This analysis is not without problems since the enlightened Buddha or the Shakyamuni does not need protection according to Buddhism. But maybe there is another explanation for Herakles continuous presence at the Buddha´s side.
One possibility is that Vajrapani is an assistant that accompanies the Buddha to carry the symbolic weapon of the Buddhist power (the Vajra) and only operates it when he is commanded to do so. In a text his presence would not always be mentioned but in visual art he would always be there, you could argue that Vajrapani's presence is not important he just happens to carry the instrument that symbolises the power of Buddhism and is always on ‘stand by’ so to say. But still currently the most used interpretation among historians is Vajrapani the protector / guard of Shakyamuni.
In the second century CE an artistic model for the Buddhist Vajrapani statue has matured under Kushan rule. Using Herakles as a template with his olive wood club, impenetrable lion pelt and muscled body, with a background in Indian Yaksa culture the fly whisk and the Vajra weapon based on the symbolic characteristics of a Diamond. Already during Gandharan time his appearance develops and his nakedness is covered with garments and his club is mostly omitted and replaced with the thunderbolt.
The role of guard is also how his position would be interpreted during his next transformation phase in China. During his stay in China the Herakles features of Vajrapani would fade but not get lost, Herakles features are even surfacing in the archaeological record among several other deities both Buddhist and with local religions in China. (ref: "Heracles in the East: The Diffusion and Transformation of His Image in the Arts of Central Asia, India, and Medieval China", I-Tien Hsing)