How it is possible to make a candala a Vaisnava?" No, that is possible. Prabhavisnave namah. Visnu is so powerful, omnipotent. He can do that. So only by Visnu mantra, by becoming a Vaisnava, one can transcend all this restriction, sociology
-Lecture on Bhagavad-Gita 1.40 -- London, July 28, 1973

यस्य वाचा प्रजा: सर्वा गावस्तन्त्येव यन्त्रिता: ।
हरन्ति बलिमायत्तास्तस्मै मुख्याय ते नम: ॥ ८ ॥
All the living entities within the universe are conducted by the Vedic directions, as a bull is directed by the rope attached to its nose. No one can violate the rules laid down in the Vedic literatures. To the chief person, who has contributed the Vedas, we offer our respect!
-Shrimad Bhagvatam Canto 3 Chapter 15 Verse 18