I think there should be permanent vault expansions available, for a much higher cost of course. Currently I'm pretty sure that many people aren't buying these items just because there's that time on them, even when they are quite cheap.
A permanent normal vault expansion for say 5 or 10 times the price of a 30 days would probably sell well. In addition, they wouldn't make cheaper 30 day expansions useless, since you could still unlock it for a month, put items that you don't think you'll need for a while there, and take them out once you finally need them (you don't need an expansion for that).
All in all, I think it would be a win-win for both the producer and the players at least for normal and guild vaults. Account vault's maybe not so much, since I'm pretty sure those sell well when there's a way to gain something from them (events basically). Even in the case of those it would be nice though. I can't claim to know exactly how well vault expansions do sell. My estimation could be wrong, but I am quite confident in it.
Also, would be nice if all the expansions were in the inventory section of the IM shop, but looking at how messy it is in general, I guess there's a reason they aren't there.
Edited by Precrush, 02 January 2016 - 05:33 AM.