I think a plain read of the license would make it clear that the "distribution" obligation occurs when you make it available on the network to your users (regardless of whether you modified it yourself or paid a contractor to do so), but I'm not a lawyer, so sometimes what looks like plain text to me can have a specific meaning that doesn't make sense to me.
Well the wording is "Notwithstanding any other provision of this License, if you modify the Program, your modified version must prominently offer[...]"
Starting with no trickery, there's a lot of circumstances where you could buy software from a vendor and confidently declare you didn't modify it. And that should be true even if you ask for certain features just for you. So there's probably a way to make the separation work.
I think a plain read of the license would make it clear that the "distribution" obligation occurs when you make it available on the network to your users (regardless of whether you modified it yourself or paid a contractor to do so), but I'm not a lawyer, so sometimes what looks like plain text to me can have a specific meaning that doesn't make sense to me.