El bullying has not made it to the RAE yet. I checked a few other popular bilingual dictionaries and one has it listed.
I think the English usage in Spanish derived from social media and even though the word exists in Spanish this is a classic example of the power of the internet. El acoso escolar is correct in Spanish. Depending on where in the Spanish-speaking world one is the correct term may be heard or el bullying which has spread rampantly via the internet.
That's a lot of topics in one response but I understand what you are saying completely. To be simple the word vaina is classified as a 'sociolect' and that is the way I analyze the usage of the word. From some people I expect it and from others because of their level of education, profession, social class not so much. For me I put in the same category as many of the local Dominican words you hear. Good to know and understand but not necessarily to use. Choice of the speaker.
Have a look at the various meanings of vaina as per Diccionario Reverso and this a very good bilingual dictionary BTW.
This is why I stated what I did in my post above and I stand by it. The usage is not as random as many say. It's the choice of the speaker and you can expect it to be used by certain speakers.