You need to be WAY more obvious when it's time to pay...

Finally got the "Model cannot be saved because you've reached table limit (20 tables)." message. I have no problem paying, and would have been more than happy to. Except, I didn't see the little blue message until I had painstakingly crafted around 40 tables. Then, clicking the "Change account plan" button destroys every change I made since I hit the 20 table mark... ouch.

We paid after figuring that out and are still using (and for the most part really liking) vertabelo. That one issue just really stung. You might consider a big modal that stops users in their tracks until they click "I'm OK losing my changes" or something.