A brahmana has to also be a vaishnava to be a guru

The fact that someone has to be out of the brahmana caste to be a spiritual teacher is obvious, but he also needs to be a vaishnava to be allowed to become a spiritual guru.

The highest goal in life or in the yoga process is to become a fully qualified servant of the highest personality of god, which is what the vaishnavas are doing, since they are following vishnu who is an incarnation of the highest personality of god. This is why a spiritual master has to be also a vaishnava to be properly allowed to act as a spiritual guru, since if he was anything else, he would just be fooling people as he could not lead them to reach the highest goal in life.

An unqualified guru can be very dangerous as he will just lead his disciples to hell, thats why there are rules when someone can be a spiritual guru and when not and two of the most important rules are that he has to be a brahmana and a vaishnava.