While giving discourse to Arjuna, Sri Krishna says that he was VAJRA among weapons.
आयुधानामहं वज्रं धेनूनामस्मि कामधुक्।
प्रजनश्चास्मि कन्दर्पः सर्पाणामस्मि वासुकिः।।10.28।।
Of the weapons,I am the Vajra [of Indra]; of the cows, I am the Wish-fullfilling Cow [of the heaven]; of the progenitors, I am Kandarpa (the god-of-love); of the serpents, I am Vasuki.
In the Chapter 10 of Bhagavad Gita, Sri Krishna compares himself with the greatest among various living/non-living beings, viz., among Adityas - vishnu;; among birds - Garuda, etc.
We have to remember that in Vedas, Indra was described to have eliminated Vritra with Vajra.
Now the question is - why did he not mention about Sudarsana Chakra, the weapon of Vishnu (or) Trishula of Shiva, which are considered to be SUPREME in Puranas, but mentioned about VAJRA, the weapon of Indra?