I was in your exact situation about 2 months ago. I looked very hard at the VTF15H, SS 18.T, FV15HP, PB13U, and the Captivator. My room is roughly 9000cubic feet and is open to the rest of the house. I was 90% set on ordering a set of the VTF's until a great deal popped up on a like new set of the CHT 18's. I figured given the (minimal) investment, I'd be crazy not to give them a try. I'm not the least bit disappointed. My room is filled with very tight, musical, quality, pounding bass. I've hit consistent 120+db at the LP with both music and movies in my room.
Very few have heard the new VS18.1 in person. Although I own the sealed version of the 18's, the output of the new ported VS's are spec'd to be far greater than that of the sealed version. I can say for sure in your room size I highly doubt you'd be dissapointed with even the sealed version of these subs - I can't even imagine the output of the ported model.
With that said, I don't think you can go wrong with the Hsu or CHT subs. I've also heard a few Hsu subs and was impressed with the overall build and sound quality. I don't have a personal preference toward sealed or ported really, as I've been impressed by subs on both sides of the fence.
I'll soon be shopping for a sub setup for my basement, which is a bit over 5k cubic feet. On my shopping list are a pair of the VS18's as well as a passive captivator. I'd really LOVE to hear these subs in person as that would probably make my decision a bit easier.
Good luck and I'm sure you'll enjoy either choice thoroughly.