I believe the intent is that you can't gain it and you can't use virtual coin; you must use Copper. That said, I don't think the restriction is phrased well.
I would prefer a short and simple phrasing for the card. Treat this as a litmus test for that. If people understand it, the job was well done. You seemed to get it right away, whereas Archetype didn't. So... I'll wait for more responses, or for a better wording (I have plenty of LONG wordings that work, but as I said, short is the goal).
What happens if I have 4 Copper and two Festivals in play? Can I buy two of these? Am I restricted to just one? Can I buy none of them?
You can buy one, using the 4 coins produced by the Coppers. Nothing in the game requires you to track where your coins came from, really (that I know of), but it isn't difficult to do so in this manner, especially since Coppers aren't played until they're actually being used. I don't really see the validity of these particular questions, or rather, I feel most players will find the right answers without outside help.
What if I play 4 Highways? Can I buy it with 0 Copper?
This is a bit more complicated and perhaps the basis for any wording issues this card might have. Still, since you spent 0 Copper on the card, it says you trash it. I'm eager to know what most people think would happen in this scenario.
The thing is, without that restriction, this card is way too cheap for the VP it gives. And even with it, I wonder if it would be dominant to only buy Copper and this card (and Estates, maybe Duchies for the 3-pile). Probably not, but I wonder.
If you pile rush estates and this, it becomes very much like a Gardens rush, except you can't use the aid of Workshop.