To become a servant of god, you have to find a spiritual master, meaning you have to find a vaishnava who is already serving god and serve him, you cannot serve god directly.
It is similar to the material world, if you want to get the best job, you cannot just start at the top, you usually have to start low and prove yourself, so that the lower workers are pleased with you and will promote you and this way over time you get promoted further until you reach the top position, it is like that in the spiritual oder as well.
The other important part on serving a vaishnava first is that he can teach you directly, where as it is very difficult to get instructions from god directly.
In rare cases where no spiritual master is available you can indeed serve god directly, but in that case god will play the part of the spiritual master and guide you from within. The normal and recommended path however is to find a vaishnava first and serve him, since it is easier and more beneficial for both. No matter the situation a spiritual master is always required.
I'm just explaining this so you can understand the principle behind it, god and gods servants are kind of one unit, you cannot gain gods favor, but treat his servants badly.
Let me give one more metaphor to explain this. If one country sends an ambassador to another country, that ambassador represents the whole country and if that other country treats him badly or even kills him, the country has basically declared war with the other country. The same way a servant of god represents the whole of god and everything you do to him you do to god, but he has to be a valid servant of god for this to apply, you cannot just pretend to be a servant of god to impress people, this in itself is a great sin.