That sounds more like AT&T being bad rather than VPN being good.

A VPN is just another link in the chain that could become a weak link. AT&T are likely inspecting your traffic to shape (prioritise) it as they see fit, for example if some chump is watching gigabyte after gigabyte of video that may be considered a lesser priority than say a messaging service. -- Your use of VPN would just impair AT&T's ability to accurately judge what your traffic is and classify it as "generic fluff".

That being said in the US didn't the FCC repeal net neutrality under the leadership of Ajit Pai, thus opening the doors for your ISPs to tier traffic into whatever fast and slow lanes they like? (E.g. cut deals with digital services to improve service quality, penalise those that refuse to pay more, etc.)