The most likely scenario is they are going to see your activity on a website and track your IP address to the VPN. Unless the VPN service provider tells them the IP address they are mapping your traffic to, it's a dead end.
The key is that the VPN provider must be trustworthy.
That's my theory. I am more than happy to be disabused of it.
HTTP has two primary pieces of information that is useful: What your IP address is and what the request is to the distance server. At a minimum they can tell what server you’re trying to get to, even if you use a proxy server, that server is sending messages to other servers. As far as encryption: All commercial encryption is a joke. All encryption. It is just plain silly to think that commercial encryption beats military encryption. If that were the case then why would the military have its own encryption? 2048 bit keys? So? Key length means not much. There are military systems that don’t even use keys. What math algorithm are you using? Is it weak? Do you really know? What is its relation to a rather large prime number? I remember reading of “largest prime number found” by universities of lengths the NSA had 30+ years ago. Primes are how encryption is cracked, not brute force against keys, so key length means nothing. Once the NSA has the right prime factored equation to crack an encryption equation they simply put that onto a chip and run it at high speed in large numbers. The military/intel agencies wouldn’t maintain extremely large data collection sytems if all everyone had to do was encrypt using commercial encryption.
That is a very big assumption that is very probably wrong.
In any event the endpoints of a VPN connection are easy to track even if the messages remain encrypted. Some of those endpoints will certainly become infested with government-developed keyloggers and other malware that will attempt to install on your PC.
If "they" think it may be worthwhile, "they" can and will track you down to physical location. The Fusion centers are very good at sifting through fields of haystacks to find a needle.
Post accordingly.
Of course they will get about 174 plausible matches on a good search and the results may be somewhat ambiguous for some posters. What with overtime limits and all, it may take several days for anyone to show up at your door.
In fact, the search for you may never get run because there are too many requests ahead of yours.