Have a look at this post from @lubos to a related query of mine a few weeks ago. You might get some useful ideas from some of the other replies there as well.
For the VAT return I found it easiest to create a new tax code for the USD transactions. This separates the ZWL from USD in many of the reports, but since Manager still converts everything to the base currency for the Summary, it’s also easy to see the ZWL equivalent of everything.
I’ll be doing my own QPD in the next few days and will hopefully be able to offer some more useful advice after I’ve worked it through myself. I think as long as you have your customers and suppliers designated in the correct currencies for their transactions, and you routinely enter the RBZ exchange rates, you should be able to extract the information you need through built-in or custom reports. If I have any great insights as I do mine I’ll be sure to share them.