Personally I would immediately "Accuse" rather than "Suspect" those maps were ripped off, due to, the absolute identical resemblance of tiles, number of colours and level layout. You've had many coincidental experiences with me but this is no way a coincidence to happen to use the same materials. And if you've had a taste of my map recreation of Toad Man to give the Game Gear map a more Sega Megadrive look (plus other mock map attempts), you'd know that any amateur artist can simply add backgrounds and modify little things like that vine changed from twisty to withered, even I could do a simple modification like that, it doesn't take a professional. The brown walls have a slightly darker colour than your version which is easily and quickly obtained by changing the contrast level in programs like Photoshop. Their lack of reply is the primary clue to suspicious activity.
I have a few suggestions to defend your work:
1. Check out,
http://home.comcast.net/~superkidicarus/comments.html - There are no comments referring to vgmaps but if you or someone could post a comment about it, you could bring some attention to visitors and players reading the comments.
2. It would be a good idea to research and quote the exact paragraph, act and subsection of the Copyright Jurisdiction to cover the rip off committed. Unfortunately I'm not an expert in law and order.
3. Take as many screenshots and source code as you possibly can from the Flip Industries web pages so that you compile proof of fraud before they have a chance to justify their actions by updating the web pages.
Good luck and may the dispute end neatly.
EDIT:
They do not stay faithful to their Mission Statement. Check out
http://home.comcast.net/~superkidicarus/reception.html . What was it they hypocritically said? "Substantial assistance will get your name in the credits".