To trade in the OG Vita to Gamestop, I need to include the USB cable, but that will render my two Vita Portable Chargers useless. The Slim Vita that I'm upgrading to already has a microUSB cable (it was basically a dinger from Amazon Warehouse Deals so everything was included), so I'm not worried about that. I would just prefer to get more use out of those chargers--not necessarily to charge the Slim Vita since they can charge other things--rather than sell/trade them for a pitiful amount.
I would assume SOME USB chargers would charge those Vita Portable chargers. Like I mentioned my cell phone charger will charge the OG vita just fine. Maybe try some of your other USB chargers and see if they put out enough power to charge them?
The charger I have is a Motorola Quick Charger that came with my Droid Turbo. Groupon had them for sale refurbished so I bought a second one and leave it at work. I think it was like $15 delivered when it was on sale. I have original iPad chargers that would NOT charge the Vita though.