Decrypting usenet headers, part 2

So the troll/spammer has changed to Google Groups and is using a VPN for the IP, but all posts have a "posting-account" header, which is encoded, and I was wondering if anyone of you guys could see what kind of format it is and how to possibly decode it?

Here are some examples:

• VaXkVAoAAADbkEFbLXXJcNV34P1KTZKR
  
• Vm0uAgoAAABvTYeieyl4GElbOkHDqJYr
  
• v_2vHwoAAACkQKbseN841UEbqgaDTAQd
  

All seem to have a "AAA" part in them that feels like it is significant.